Mr Hosokawa, a colourful figure who likes to remind interviewers that he is descended from an ancient family of warlords, was delighted with the result, announcing that he would increase the number of candidates in the national elections. But his opponents have been calling his reformist credentials into question, pointing out that he has admitted links to the scandal-ridden Sagawa Kyubin company that felled Shin Kanemaru, the LDP godfather, last year.
The JNP, established last year but only now emerging into the light, joins two other new groups, the Shinsei party of Tsutomu Hata, and the smaller Sakigake party, all of which have split from the LDP to try to form conservative-oriented alternatives to the ruling party's 38 years in power. Mr Hosokawa has cleverly avoided committing himself to any coalition pact, hoping to increase his bargaining power between the other opposition parties and the Liberal Democrats after the poll.
The rise of these right-wing opposition parties, who will follow broadly similar economic, foreign and defence policies to those of the LDP, is likely to cut into support for the Socialist Party, the main opposition party up to now. In Sunday's vote the Socialists suffered a humiliating defeat, losing more than half of their 32 seats and gaining just 13 per cent of the vote, compared to 24 per cent in the last elections in 1989.
The Liberal Democratic Party also failed to do as well as it hoped in the local polls, which were closely followed for indications of how voters might react in the national elections in three weeks' time. Although the ruling party gained two seats for a total of 44, its share of the vote was 31 per cent, a disappointingly small improvement on a catastrophic showing in the last local polls in 1989.
But yesterday Mr Hosokawa was the focus of attention. 'To be honest, it is too good to be true,' he said, after the new party's 20 seats were announced. He said he now intended to field some 70 candidates for the general elections, 10 more than planned, and that his party would approach the vote 'with its sails filled with wind'.
Mr Hosokawa, 55, comes from a long line of feudal lords stretching back five centuries. His grandfather was one of Japan's wartime prime ministers, and his family wields immense power in his home town of Kumamoto, in southern Japan. After graduating from a Tokyo university with a law degree, he joined the Asahi newspaper, before being elected to the Upper House of parliament in 1971.
But power in Japanese politics often rests in the provinces, and in 1983 he returned to become a local hero: he ran for, and won, the post of governor of Kumamoto prefecture. After a conflict within the LDP over the next step in his career, he withdrew to set up the Japan New Party in the summer of 1992, jumping on the trendy 'political reform' bandwagon.
However, earlier this year he was forced to admit to a Japanese magazine that he had been receiving political donations from Sagawa Kyubin, a delivery company accused of political bribery and links with organised crime. In addition Sagawa lent him 100 million yen (pounds 600,000) to help him buy a house in Tokyo. And despite his public calls for more open politics, within his own party he treats his colleagues 'like feudal vassals', according to Tetsuhisa Matsuzaki, who recently left the JNP.