A spiral spring (5) has an inner circumferential end (5a) fixed to a rotor (3) and an outer circumferential end (5b) fixed to a plate (8) of a housing (2), and biases the rotor (3) in one direction with respect to the housing (2). A projection (13) protrudes from the plate (8), and stops radially outward expansion of an outermost winding (5c) of the spiral spring (5). A clip (6) has a first portion (6a) coming in contact with the outermost winding (5c) of the spiral spring (5), and is attached to the projection (13) by elastic force of the clip (6).