Well like all government agencies, there are multiple actors. Three are important in particular, often reffered to as the Iron triangle for the impressive control they hoard on economic matters, this is the Political party, The Ministry of Finance, and buisness. Buisnesses in Japan have a larger influence in economic matters than in other western democracies (far more than in the United States). This helped large buisnesses flourish during the boom years. The second was the political party, or the ruling Liberal Democratic Party, which, before 1994 had ruled for nearly 50 years. But the coalition that unseated the LDP, under Morihiro Hosokawa, was quickly culled and the LDP returned to power in 1996 under HAshimoto. Because of the nature of the Japanese election system, Members of the Diet were very rarely unseated, and had long lives in the Diet. This allowed Buisnesses to work with the same people for long periods of time and ensured continued patronage.
But the political games thay may occurred in the LDP and Buisnesses pales in comparison to the power of the main agent; the Ministry of Finance. The Bureaucratic power it wields is absolutely astounding, in comparision to other bureacacies in the world. Working in the ministry of Finance was the Job above all jobs in Japan, they only take the top graduates from the University of Tokyo. They often worked as how they saw fit, thinking they know what is best for the Japanese economy. there is a story once of a finance minister who didn't do what the MOF wanted, and said something on television that was contrary to the ministry's policy. He had already been warned about this, but he persisted in the view. After the interview, when the Minister was on a shinkansen going back to his home district, a high ranking official from the ministry set up a interview and publically berated the minister, his superior, about the policy. I think that minster quit a few months after. That would never happen in any other democracy. There are cases when high ranking bureacratic officials just don't listen to the ministers and do what they think is best.
The last decade has seen the power of the Iron triange weaken, but it still commands significant power. Consitently progressive initatives have been killed by the triangle, although reformers have been able to make some significant victories. The Power of the Bureacracy has been constrained, especially in the light of several very embarassing scandals. It nolonger commands the respect as it did once before, but still acts with a fair bit of arrogance, compared to western bureacracies.
I hope that helps