To provide a planetary roller speed changer capable of sufficiently absorbing variations of rotation angular velocity deriving from output variations of a motor and of offering smooth output rotation with less variations of angular velocity, even if a planetary roller speed changer is of a one-stage constitution type.

A means (such as one way clutch 11) for absorbing the variation of rotation angular velocity is provided in the planetary roller speed changer, whereby the variation of rotation angular velocity is prevented from being transmitted to an output shaft 5 so as to obtain smooth output rotation, even when a motor with a large variation of angular velocity such as a stepping motor 20 is employed as a driving source.