Wage inequality has presumably been increasing due to the expansion of non-regular employment in Japan. However, limited studies have evaluated this, since the majority of previous studies used the Basic Survey on Wage Structure, which did not capture employment status until 2005. Herein, we use data from the Employment Status Survey conducted in 1992, 1997, 2002, 2007, and 2012. This survey evaluates annual earnings by class, the range of which varies depending on the survey year. Therefore, simply unifying the differences per earnings class range may lead to an overestimation of inequality. To correct for this problem, the inverse transform method is used to generate random numbers with a specific function within the class of earnings. The calculation shows that for men, wage inequality increased between 1992 and 2012, while for women, it decreased after 1997. The analysis methods include sub-group and regression-based decomposition. Sub-group decomposition analysis shows that changes in the age structure and the education composition of workers and changes in the age-specific wage curve does not contribute to the widening of wage inequality among men.Regression-based decomposition analysis shows that for men, the impact of non-regular employment on wage inequality at each point in time is not as great as that of other variables. However, changes in the share of non-regular employees and their relative wages are factors that have contributed most to the increase in wage inequality over the past 20 years. On the other hand, non-regular employment is the most significant contributing factor to wage inequality at each point in time, for women. Although, for women, the share of non-regular employees increased significantly, the change in employment status does not cause wage inequality because of the increase in relative wages of non-regular employees
