According to the research by Ministry of Health, Labour and Welfare, 231,384 couples got divorced last year. The number of marriages was 660,594 last year. So divorce/marriage = 35%. The number of divorces is decreasing slightly these days.
Figure 1: Number of divorces (1985-2013, Japan)
Ministry of Health, Labour and Welfare also researches the number of divorces by cohabitation period. As you may know, short-time period couples are likely to get divorced rather than long-time period couples.
Figure 2: Number of divorces by cohabitation period (1985-2013, Japan)
Figure 3: Number of divorces by cohabitation period (in cases of less than 5 years, 1985-2013, Japan)
Source is Ministry of Health, Labour and Welfare. 人口動態調査