An anchor position adjustment device configured to adjust a position of an anchor supporting a seatbelt, the anchor position adjustment device comprising: a guide rail configured to be fixed to a vehicle body and having a plurality of engagement recess portions formed to be arranged in a first direction; a moving member attached to the guide rail in a slidable manner in the first direction and having the anchor attached thereto; a lock pin provided in the moving member such that the lock pin is able to move in an inserting/pulling direction with respect to the engagement recess portions; a 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 in a direction intersecting with the inserting/pulling direction from the lock pin; and an operation member attached to the moving member in a slidable manner, wherein 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, a release inhibition surface (97, 97A, 97B) inhibiting the lock pin from being pulled out from the engagement recess portion in a pre-slide state before the operation member slides, and a slide restriction surface restricting sliding of the operation member when the release pin abuts the slide restriction surface, and wherein in a slide direction of the operation member, when a direction in which the operation member slides from the pre-slide state and the lock pin is pulled out from the plurality of engagement recess portions is referred to as an operation direction, characterized in that the release inhibition surface (97, 97A, 97B) is positioned on a side opposite to the guide rail with respect to the release pin and extends to a position facing the release inclination surface, whereby a distance between the release inclination surface and the release inhibition surface (97, 97A, 97B) in a direction perpendicular to the release inclination surface increases toward the operation direction, and the release inclination surface and the release inhibition surface (97, 97A, 97B) are connected to the slide restriction surface. a guide rail configured to be fixed to a vehicle body and having a plurality of engagement recess portions formed to be arranged in a first direction; a moving member attached to the guide rail in a slidable manner in the first direction and having the anchor attached thereto; a lock pin provided in the moving member such that the lock pin is able to move in an inserting/pulling direction with respect to the engagement recess portions; a 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 in a direction intersecting with the inserting/pulling direction from the lock pin; and an operation member attached to the moving member in a slidable manner, wherein 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, a release inhibition surface (97, 97A, 97B) inhibiting the lock pin from being pulled out from the engagement recess portion in a pre-slide state before the operation member slides, and a slide restriction surface restricting sliding of the operation member when the release pin abuts the slide restriction surface, and 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, a release inhibition surface (97, 97A, 97B) inhibiting the lock pin from being pulled out from the engagement recess portion in a pre-slide state before the operation member slides, and a slide restriction surface restricting sliding of the operation member when the release pin abuts the slide restriction surface, and wherein in a slide direction of the operation member, when a direction in which the operation member slides from the pre-slide state and the lock pin is pulled out from the plurality of engagement recess portions is referred to as an operation direction, characterized in that the release inhibition surface (97, 97A, 97B) is positioned on a side opposite to the guide rail with respect to the release pin and extends to a position facing the release inclination surface, whereby a distance between the release inclination surface and the release inhibition surface (97, 97A, 97B) in a direction perpendicular to the release inclination surface increases toward the operation direction, and the release inclination surface and the release inhibition surface (97, 97A, 97B) are connected to the slide restriction surface.