A speed reducer includes a casing, a crankshaft, and a speed reducer main body. The crankshaft has a rotation support part rotatably supported by the casing and an eccentric part eccentric with respect to the rotation support part. The speed reducer main body has an engagement part engaged with an outer circumferential surface of the eccentric part, decelerates rocking rotation of the eccentric part received by the engagement part, and outputs the decelerated rotation to outside. The crankshaft is provided with a flange part protruding to an outward side in a radial direction from an end part of the rotation support part on the eccentric part side and abutting an end surface of the engagement part in an axial direction. The flange part is formed such that a center thereof coincides with a center of the eccentric part in a direction orthogonal to the axial direction.