SPEED REDUCER AND CONSTRUCTION MACHINE

A speed reducer (1) according to one aspect of the present invention includes: speed reducing units (4) for decelerating a rotational driving force of an electric motor (2) and transmitting the decelerated rotational driving force to a rotational driving unit; and a ring gear (5A) meshing with a gear decelerated by a speed reducing unit (4) on the electric motor (2) side in a direction of a rotational axis of the electric motor (2). The ring gear (5A) is provided with a water-cooling channel (6) that communicates cooling water (W). Another ring gear (5B) rotatably supported on the ring gear (5A) is provided. The water-cooling channel (6) includes a first groove (63) having a small depth and a second groove (64) having a larger depth than the first groove (63) and extends in a C-shape as the ring gear (5A) is viewed from the direction of the rotational axis. The ring gear (5A) is provided with a motor flange (23) mounted to the electric motor (2). The motor flange (23) is provided with an inlet port (61) and an outlet port (62) connected to the water-cooling channel (6).