A speed reducer includes a helical gear, an eccentric shaft and a slider plate. The speed reducer also includes a transmission gear and an output gear body. The transmission gear is supported at a first support part, transmission gear spinning is restricted by engagement with the slider plate, and the transmission gear revolves by the helical gear rotating with the eccentric shaft. The output gear body rotates by the transmission gear revolving. The transmission gear includes a pair of restricting protrusions protruding toward the slider plate side. The restricting protrusions have engaging surfaces that touch the slider plate wherein the engaging surfaces oppose the slider plate in a radial direction of the transmission gear. Surfaces at opposite sides of the restricting protrusions from the slider plate are formed, as seen in the axis direction of the transmission gear, to be convex to the opposite sides thereof from the slider plate.