A driving-side rotatable body includes a driving-side inner gear, which has an axial extent that does not overlap with an axial extent of a driven-side inner gear of a driven-side rotatable body. A driven-side outer gear and a driving-side outer gear of a planet gear are meshed with and are driven together with the driven-side inner gear and the driving-side inner gear, so that the planet gear changes a relative rotational phase between the driven-side rotatable body and the driving-side rotatable body. A length of a tooth contact between the driven-side inner gear and the driven-side outer gear measured in a direction of a tooth trace thereof is set to be longer than a length of a tooth contact between the driving-side inner gear and the driving-side outer gear measured in a direction of a tooth trace thereof.