An anchor position adjustment device includes a guide rail having a plurality of engagement recess portions formed to be arranged, a moving member attached to the guide rail, a lock pin-provided in the moving member, a first biasing member biasing the lock pin in an inserting direction of the lock pin with respect to the engagement recess portions, a release pin extending from the lock pin, and an operation member attached to the moving member in a slidable manner. The operation member has a release inclination surface guiding the release pin in a pulling direction of the lock pin with respect to the engagement recess portions in accordance with sliding of the operation member, and a release inhibition surface inhibiting the lock pin from being pulled out from the engagement recess portion in a pre-slide state.