A speed reducer includes an outer tubular member, internal tooth pins, oscillating gears and a carrier block. The outer tubular member has pin grooves on an inner circumferential surface thereof. The internal tooth pins are rotatably placed in the pin grooves. The oscillating gears have external teeth smaller in number than the pin grooves, and the oscillating gears are oscillatorily rotatable with the external teeth engaging with the internal tooth pins. The carrier block is cooperatively connected to the oscillating gears such that the oscillating gears are allowed to oscillatorily rotate relative to the carrier block and restricted from rotating on own axis relative to the carrier block. A displacement restricting member configured to restrict the internal tooth pins from being displaced inwardly in a radial direction of the outer tubular member is disposed between the outer peripheries of the oscillating gears adjacent to each other.