This study aims to analyze the gap between desired and actual number of children theoretically. Our model follows Becker（1965）, but we divide the time devoted to child-raising activities between men and women. Using our model, we analyze the effect of the women's wages, the men's wages, and the prices of childcare serves on the demand for children. Some variable increases the demand for children, so it is possible that the variable causes the gap between desired and actual number of children. Our model shows that the high prices of childcare serves and the low men's wages cause the gap.