The aim of this paper is to compare in an empirical way the male/female wage gap differential between the following two countries : France, an industrialized country where the disparities between sexes are outstandingly small, and Japan, where they are particularly large. It is also proposed to explore the reasons why the male/female wage gap is so large in Japan in comparison with France. It has been designed as follows. In section 1, we begin by comparing the general features of labour markets in Japan and France. When it comes to the gender wage gaps, it is necessary to take the general structure of wages and the way the labour markets function in each country into account. Based on this comparative description, we will then propose some hypotheses, which will subsequently be tested. In section 2, the econometric method used here, the Juhn-Murphy-Pierce decomposition method, will be presented in detail. In section 3, Concrete analysis will be carried out on individual micro data collected in Japan and France. Decomposion analysis is carried out in order to identify the types of factors mainly responsible for the the gender wage gap differentials between the two countries. Factor decomposition analysis focusing on full-time workers provides the first model. In the second model, occasional part-time workers are introduced into the model. We will then carry out a specific analysis on the male/female wage gap differential between France and Japan in three age groups corresponding to the various phases in the female life cycle. We shall conclude by summarising the most significant results obtained and attempting to deduce some positive policy implications.