The conventional devices for improving a pelvic tilt and movement of a sacroiliac joint have been used in a supine position and in a seated position. Some of these conventional devices cannot provide sufficient mobility. The present invention provides an exercise assist device which transitively moves a sacroiliac joint in a lateral decubitus position. The following will describe Example in the context of a left lateral decubitus position. The upper right leg is placed on a stand (1) 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 (2). Since a posterior side of a lower left sacroiliac joint (9b) is slightly opened, a sacrum (6) slides backward via a right hip joint (10a). This improves mobility of the lower left sacroiliac joint (9b).
