We reported the major findings of our research based on our own academic achievement tests towards elementary school and junior high school pupils in 2002. We then pointed out the fact that the differences of achievement between social groups have been expanded. Nowadays, that issue is seen to be one of the most serious educational problems in contemporary Japan. Although the differences of various educational outcomes such as academic achievements or educational aspirations between social groups are always emphasized, it is surprising that they seldom discuss about the ways in which those differences could be made smaller. I myself have been exploring the issue in these several years. In this paper, I will describe the progress and the future directions of our academic exploration on this particular educational issue. In section 2, I will give some consideration on the basic concepts such as 'gakuryoku' (academic achievement) and 'gakuryoku kakusa' (collective difference of academic achievement) and locate the existence of the schools that are actually reducing the differences in the context of the theory of effective school. That theory or research trend has been developed in the U.S.A. and the U.K. in these three decades. The concept of effective school is related to a kind of school that can reduce the differences of academic achievement between social groups such as social classes or ethnic groups. In section 3, the findings of our collaborative research carried our in 2002 will be shown and the actual contents of effective schools found out in the research will be discussed. In those schools (one elementary school and one junior high school), the averages of achievement of the children are pretty high and the ratios of low-achievers remain fairly small. The overall efforts of the school towards guaranteeing the minimum level of achievement for all the children seems to bear fruit sufficiently. In section 4, I will tough the contents of our on-going research project carried out in Osaka. The aim of the project is to find our various kinds of effective school in Japan and to draw common characteristics of those schools. We provisionally present seven factors that can contribute to make a Japanese school effective: not to make the children rough, to develop the good relationship among the children, school management emphasizing teamwork among members of staff, positive and practice-oriented school culture, collaboration with parents and local community, internal system guaranteeing the minimum level of achievement, existence of leaders and leadership. In section 5, I will consider several issues I order to prospect future development of research on school effectiveness in Japan. The following is the issues I will pick up: development of appropriate achievement tests, development of appropriate indication of family backgrounds, planning of longitudinal research on school effectiveness, necessity for research on the process of school improvement.