The first objective of this study was to develop a musculoskeletal simulation model to analyze the muscle load of a pregnant woman. The second objective of this study was to investigate the muscle load on the Erector Spinae numerically using the developed simulation model. An experiment to validate the simulation was also conducted. The following findings were obtained: In the simulation of a standing non-pregnant woman, the muscle force is relatively small when the bending angle of the body is -20 to 0 degrees, while it gradually increases as the bending angle increases from 0 degrees. In the simulation of a pregnant woman, on the other hand, the muscle force increases monotonously for the entire range of the bending angle. In the simulation of a straight standing pregnant woman, the muscle force becomes 2 to 4 times larger than a nonpregnant woman. The validity of the simulation was confirmed since the experimental values of the muscle force were almost within the range of the simulated values. In the simulation of a seated pregnant woman, the muscle force becomes significantly smaller than that in the case of a standing pregnant woman when the backrest of the seat is lent back more than 10 degrees. The ratio becomes less than 0.5 when the backrest is lent back more than 20 degrees.