The purpose of this study is to account for the increase in non-regular workers, namely, part-time and dispatched workers, in the Japanese economy from the early 2000s. Our contribution is that we use a firm-level panel dataset extracted from an administrative survey and distinguish between the short-run and long-run determinants of non-regular labor demand. Using the estimated parameters of the labor demand function, we decompose the rate of increase in the macroeconomic non-regular worker ratio into determinant factor contributions. Our major results can be summarized as follows. First, the firm-level determinants of the demand for part-time and dispatched workers significantly differ. Second, our results suggest that the non-regular job creation stimulated by the increased female labor supply plays an essential role relative to direct demand-side factors. Third, the microeconomic demand conditions for non-regular labor are widely dispersed among firms. Neither the demand factors examined in this study nor industrial differences can explain this heterogeneity.