By announcing that he will resign as prime minister of Japan, Noboru Takeshita is merely demonstrating his grasp of the obvious. Unhappily, that grasp is not quite as firm as it ought to be.

What should be obvious is that Takeshita cannot govern effectively. Not when three of his cabinet ministers already have resigned in the scandal involving massive payments from the Recruit publishing and telecommunications conglomerate to politicians and others. Not when parliament has ground to a virtual halt. Not with opinion polls showing Takeshita has the approval of only 3.9 percent of the public, making him Japan's most unpopular post-war prime minister.What seems to have eluded Takeshita is the impact of his announcement, which automatically turns him into a lame duck and thus erodes what little influence he had left.

Consequently, instead of leaving the date of his departure up in the air, Takeshita ought to hasten it.

His excuse for waiting is that parliament has not yet passed the 1989 budget and he wants to make sure it does so before leaving office. But parliament, or the Diet, has been stalled by a boycott of opposition parties demanding that former Prime Minister Yasuhiro Nakasone testify about the Recruit scandal.

Meanwhile, the current scandal is a telling symptom of what ails Japanese politics. This is the fourth political scandal in Japan since World War II. All of them involved efforts by big business to buy politicians. When he leaves, Takeshita will have served only about 17 months, making him the shortest-serving premier in more than 30 years.

What's needed is clearly not just a change of government but a thorough overhaul of political ethics in Japan. The polls on Takeshita's lack of popular support show that the Japanese people want a higher standard of political morality than many of their elected leaders are giving them.