A bushing member includes: a large diameter portion which is inserted on a radially inner side of a coiled segment of an assist spring and includes a straight portion which extends in an axial direction; and a small diameter portion which is located on a radially inner side of a housing and has an outer diameter that is smaller than an outer diameter of the large diameter portion. The coiled segment includes a contact portion that contacts the straight portion at a radially inner surface of the contact portion. In an axial direction, a location of an end of the straight portion, which is located on one axial side where a driven shaft is placed, coincides with a location of an end of the contact portion located on the one axial side, or is on the one axial side of the location of the end of the contact portion.