The objective of this study was to develop the method of musculoskeletal simulation for a swimmer attaching a competitive swimwear. For the simulation, the body geometry and joint motion were put into the swimming human simulation model SWUM in order to calculate the distributed fluid forces acting on the whole swimmer's body. The distributed fluid forces were put into the musculoskeletal model. In addition to the fluid forces, the forces induced by the tension of the swimwear were also considered. The distribution of the forces acting on the swimmer was calculated from the stress distribution of the swimwear. Since the swimwear targeted in the present study was a half spat type, the forces by the swimwear acted on three segments; the right and left thighs, and the pelvis. Therefore, the ratio of one distributed force to these three segments had to be determined. In order to determine this ratio, the deformation of the swimwear measured in an experiment was used. As an example, the simulation for the crawl stroke was conducted. The results showed the time-varying effect of the swimwear on the muscle activity during swimming.