A speed reducer comprising: an input rotating part; an output rotating part disposed coaxially with the input rotating part; a fixed member; a first rolling element engaging groove disposed in the input rotating part and provided along a circle having a curvature center eccentric from a rotation center of the input rotating part and the output rotating part; a second rolling element engaging groove disposed in the fixed member and provided along a corrugated curve alternately intersecting a pitch circle having a curvature center on the rotation center; a plurality of rolling elements (4, 25) engaging with the first rolling element engaging groove and the second rolling element engaging groove facing each other in an axial direction; characterised in that a plurality of pockets are disposed in the output rotating part and holding the plurality of rolling elements (4, 25) in a circumferential direction between the first rolling element engaging groove and the second rolling element engaging groove in the axial direction, wherein the input rotating part comprises: an input shaft; an eccentric cam formed separately from or integrally with the input shaft; an input member in which the first rolling element engaging groove is formed, the input member having a cylindrical part and a disk part extending in an inner diameter direction from an end of the cylindrical part, and the first and second rolling element engaging grooves (13, 16) are formed on axially opposing side surfaces of the input member and the fixed member; and a rolling bearing mounted between a cylindrical outer peripheral surface of the eccentric cam and an inner peripheral surface of the cylindrical part of the input member. an input rotating part; an output rotating part disposed coaxially with the input rotating part; a fixed member; a first rolling element engaging groove disposed in the input rotating part and provided along a circle having a curvature center eccentric from a rotation center of the input rotating part and the output rotating part; a second rolling element engaging groove disposed in the fixed member and provided along a corrugated curve alternately intersecting a pitch circle having a curvature center on the rotation center; a plurality of rolling elements (4, 25) engaging with the first rolling element engaging groove and the second rolling element engaging groove facing each other in an axial direction; characterised in that a plurality of pockets are disposed in the output rotating part and holding the plurality of rolling elements (4, 25) in a circumferential direction between the first rolling element engaging groove and the second rolling element engaging groove in the axial direction, wherein the input rotating part comprises: an input shaft; an eccentric cam formed separately from or integrally with the input shaft; an input member in which the first rolling element engaging groove is formed, the input member having a cylindrical part and a disk part extending in an inner diameter direction from an end of the cylindrical part, and the first and second rolling element engaging grooves (13, 16) are formed on axially opposing side surfaces of the input member and the fixed member; and a rolling bearing mounted between a cylindrical outer peripheral surface of the eccentric cam and an inner peripheral surface of the cylindrical part of the input member. an input shaft; an eccentric cam formed separately from or integrally with the input shaft; an input member in which the first rolling element engaging groove is formed, the input member having a cylindrical part and a disk part extending in an inner diameter direction from an end of the cylindrical part, and the first and second rolling element engaging grooves (13, 16) are formed on axially opposing side surfaces of the input member and the fixed member; and a rolling bearing mounted between a cylindrical outer peripheral surface of the eccentric cam and an inner peripheral surface of the cylindrical part of the input member.