A shoulder anchor position adjusting device is provided in which assembly of a position adjusting mechanism, which adjusts a position of a shoulder anchor, is easy. The shoulder anchor is fixed to a slider. The slider is slidably mounted to a slide rail by the position adjusting mechanism which has a lock pin and a release lever. The release lever is placed on the slider in which a stepped groove is formed. The release lever is swingably held due to engagement of an anchor plate of a cover case with a cut-out groove. A flange portion is formed at the lock pin, and is supported by a step forming portion of the stepped groove. Due to a coil spring, a shaft portion of the lock pin is inserted through an insertion hole of the slider and is fit into an engagement hole formed in the slide rail.