A seat lifter includes: a connecting member that connects a base member and a seat frame that form a seat and swings in response to external rotating force to raise or lower the seat frame; a first gear provided in the connecting member; a second gear that is provided such that an axial direction thereof is parallel to an axial direction of the first gear and meshes with the first gear; and an operating lever that applies rotating force to the second gear. The teeth of at least one of the first and second gears have such a shape that a tooth thickness of the teeth gradually decreases from one end toward the other end in the axial direction, and the first and second gears mesh with each other by approaching each other in the axial direction.