In this paper, I quantify the effects of the increase in part-timers on the flattening wage Phillips curve in Japan. Specifically, I formulate the New Keynesian DSGE model which explicitly incorporates two different types of labour forces, which are fulltime labour and part-time labour, and their unemployment considering the structural change of the increase in part-timers. I employ estimation with Japan’s data using Bayesian techniques and obtain plausible results for structural and policy parameters. Then I do a counterfactual simulation without the structural change and compare it with baseline estimation. I find that roughly 30% of the wage Phillips curve flattening in Japan has been ascribed to the increase in part-timers.