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. The driven-side rotatable body supports the driving-side rotatable body from a radially inner side of the driving-side rotatable body at a location, which is radially outward of the driven-side inner gear.