A speed reducer includes a gear, a pinion gear, a movable portion holding the gear so as to be rotatable around a first axis, and a movable portion holding the pinion gear so as to be rotatable around a second axis non-parallel to the first axis. The gear has an annular power transmission portion surrounding the first axis on one side in a direction along the first axis. The pinion gear has a power transmission portion facing the power transmission portion and transmitting rotational torque on an outer periphery. The movable portion has a bearing portion holding the pinion gear inside the power transmission portion and a bearing portion holding the pinion gear outside the power transmission portion.