A gear reducer has a helical gear, an eccentric shaft that is joined to the helical gear and has a first supporting portion that is offset in a rotation radial direction with respect to a rotation shaft of the helical gear, and a slider plate that is disposed at a radial direction outer side of the eccentric shaft. Further, the gear reducer has a transmitting gear that is supported at a first supporting portion, and whose rotation around its own axis is restricted due to the transmitting gear being engaged with the slider plate, and that revolves due to the helical gear rotating together with the eccentric shaft, and has an output gear body that rotates due to the transmitting gear revolving. The transmitting gear has a pair of restricting projections that project-out toward the slider plate side, and the slider plate is disposed between the pair of restricting projections.