FOR the first 75 years of its history, the Sori Kantei, official home of Japan’s prime ministers, was one of the least secure official residences in the world. A brief name check, a casual glance at any kind of identity card, the issuing of a pass, and you were in — without so much as a glance inside pockets, bags or camera cases.

“We always had the security screening equipment,” says one official, “but until recently they just didn’t seem necessary.”

Then, two weeks ago, came the bombings in Madrid, which were heard as loudly in Japan as anywhere. Since then, police patrols at Japanese railway stations have been doubled, there are luggage searches on the bullet train, and the litter bins on the subway have been sealed up with apologetic notices.

At the Prime Minister’s residence, the X-ray machine has been brought out of mothballs, and visitors must pass through a metal detector. By the standards of paranoia displayed at Downing Street or the White House, this is mild, somewhat inefficient, stuff. But for Japan it is theatre that was unscripted when the occupant of this building, Japan’s 56th Prime Minister, Junichiro Koizumi, took office in his slightly quirky, seemingly inoffensive way.

When he entered the Kantei, in an amazing political upset in April 2001, he inherited a Government that liked to see itself as a lead actor but was little more than a bit-part player. Japan, the popular sneer had it, was an economic giant trapped in the body of a diplomatic pygmy.

The country provided foreign aid, more than any other country in the world. But the nitty-gritty of “hard” diplomacy — the brokering of peace deals, the dispatch of peacekeepers, the risks as well as the glory — were left to others. When the heads of the G7 (or G6, plus whoever happened to be the temporary occupant in Tokyo) met at their international get-togethers, Japan’s Prime Minister was always the odd one out, lingering uncertainly on the edges of the group photograph and unable even to make small talk, let alone hold a conversation of substance.

Under Koizumi so much has changed. After a generation in the margins of international society, Japan is now at the diplomatic top table, and a target for terrorism. Its delegates shuttle between Tokyo and Beijing for the six-party talks on North Korea’s nuclear programme. Its Prime Minister is proud to be on first-name terms with the US President. A thousand of its soldiers, sailors and airmen are involved in Iraq, including several hundred on the ground.

Japan now finds itself more engaged with the outside world than at any time since the Second World War. And, as the metal detectors at the Kantei acknowledge, it is a position that brings tremendous risks as well as potential rewards. “When I get home (at night) I feel I must never crumble under the pressure and the tension,” Koizumi told The Times in a rare interview this week. “I must survive that.”

Koizumi is a remarkable Japanese Prime Minister in several respects, beginning with his simple physical appearance. He is above average height for a Japanese of his age, but surprisingly dainty in person, with a slim build and long, carefully manicured fingernails. Many Japanese men of 62 dye their hair, but Koizumi sports a gravity-defying, centre-parted perm.

Rather than the measured placidity of the typical Tokyo politician, he speaks like a man on the attack, in staccato sentences punctuated by chopping motions of his hands. And his physical distinctiveness is matched by his way of life.

Japan’s media do not in general rake over their leaders’ pasts, and little is known about Koizumi’s personal life. He married in his thirties and divorced after a few years; it is said that he has never met his third and youngest son, born after his separation. His formidable elder sister, Nobuko, reportedly organises his personal life for him.

He loves opera, film and the Japanese theatrical art of kabuki, “but when I go someone will always criticise me for taking time off when times are so tough”. He sleeps only four or five hours a night, and lies awake for three more (“I try to avoid picking up a book, I just lie there with my eyes shut”). As far as is known, for more than 20 years he has lived alone.

He was criticised after seeing a film, Seabiscuit, for neglecting his duties, then his critics were criticised. That popular support, he said, has enabled him to escape occasionally to the opera or to read historical novels instead of only the inevitable briefing papers. It is also that direct line to the public that has enabled him to break down the faction system that had crippled decision-making in postwar Japan.