Speed reducer

A speed reducer comprising:

• a casing configured to surround a main axis;
• an internal gear integrally formed with the casing;
• an external gear meshing with the internal gear;
• an eccentric body for oscillating the external gear;
• a carrier rotatable relative to the casing;
• and
• a main bearing configured to rotatably support to the carrier relative to the casing,
• wherein the internal gear includes an internal gear body integrated into the casing, and a plurality of internal tooth pins configured to mesh with the external gear,
• wherein the plurality of internal tooth pins is configured to rotatably be arranged in pin grooves formed on an inner peripheral surface of the internal gear body,
• wherein the main bearing functions as a plain bearing having an inner sliding surface and an outer sliding surface that are in contact with each other,
• wherein an inner peripheral surface of a metal ring being rotatable integrally with the casing functions as the inner sliding surface,
• wherein an outer peripheral surface of the carrier functions as the outer sliding surface,
• wherein the casing and the carrier are made of resin,
• wherein, in a direction extending along the main axis, the metal ring and the plurality of internal tooth pins is positioned to overlap each other,
• wherein the internal gear body is made of resin, and
• wherein the plurality of internal tooth pins, the metal ring, and the carrier are made of a material that has a higher thermal conductivity than the resin of the internal gear body.