ELITE members of the Japanese establishment are breathing a little harder today at the thought that, barring unexpected developments, self-admitted maverick former governor Morihiro Hosokawa will soon become prime minister. It must come as something of a shock. Japan is a conformist society which passionately believes in the adage that ''the nail which sticks out gets hammered down''. Mr Hosokawa is the nail which came back out of the wood. ''The root cause of our structural problems is that we have not had political change,'' Mr Hosokawa said this year. ''For over 50 years, we have had a kind of pseudo-democracy in practice here in Japan.'' It is safe to assert that Japan has not had a prime minister since 1945, or even since the start of Meiji democracy in the 1890s, capable of uttering such a heretical, thoroughly unJapanese thought. Now the seven opposition parties - which since they half-won the general election have displayed an uncommon degree of accord - have decided the heretic should be their prime ministerial candidate next week. Unlike the majority of post-war prime ministers, Mr Hosokawa is merely 55. Only Toshiki Kaifu was slightly younger when taking office. But, born in 1938, Mr Hosokawa does remember the deprivation of the post-World War II years - an experience which many of the younger advocates of reform completely missed. After graduating in law from Sophia University in Tokyo in 1963 and five years as a journalist, Mr Hosokawa began a decade of conventional political development. In 1971 and 1977 he was elected to represent the Liberal Democratic Party (LDP) in the upper House of Councillors, and held a variety of party posts. His only administrative experience in the national government came when he was a vice-minister at the Ministry of Finance in 1975-76. But it was when he started to tangle with the national Government that Mr Hosokawa's maverick instincts got a boost. Elected governor of Kumamoto prefecture in Kyushu in 1983, and re-elected in 1987, Mr Hosokawa quickly learned Japan was, after all, the world's most successful centralised command economy. Among his formative experiences was the discovery that to move a bus stop 10 metres in Kumamoto, permission had to be gained from the Ministry of Transport in Tokyo. There is justice and irony in the fact Mr Hosokawa's colleagues have chosen him as prime minister. It is appropriate that the man who first broke away from the LDP - rather than the more recent defectors - be made leader. The political and electoral reforms that the new government is pledged to institute have long been top of Mr Hosokawa's agenda. Yet, in a very Japanese touch, the man who will now seek to change Japan's often feudal politics, is himself best known in Kumamoto as the descendant of a feudal family which once treated the region as their fiefdom.