An exercise assist device transitively moves a sacroiliac joint in a lateral decubitus position. The following description is provided in the context of a left lateral decubitus position. The upper right leg is placed on a stand in the lateral decubitus position, and the upper right leg is then extended transitively backward with a sacral area supported by a sacrum support pad. Since a posterior side of a lower left sacroiliac joint is slightly opened, a sacrum slides backward via a right hip joint. This improves mobility of the lower left sacroiliac joint.