The present study aims at analyzing the mechanism of increasing mobility related to work-trips in the Tokyo Metropolitan Region. Most of the preceding studies on work-trips in urban areas have analyzed trips from suburban areas to the central cities. This study analyzes not only an increase in those trips but also its repercussions on work-trips within suburban areas. The conclusions are as follows: 1) The increase in work-trips stems from the lack of residential workers for the increasing number of jobs available in some municipalities. 2) An increase in trip workers relationally appears both in a place of employment and in a residential place. Furthermore, an increase in trip workers who live in some municipalities creates an increase in trip workers employed in the same municipalities. Therefore, there is a spread effect in the increase in work-trips. 3) There is a direction to the spread effect. The author defines "Integrated Population Density" to analyze population mobility, and proves that the spread effect proceeds in an order messured by the inte-grated population density.