In a drive cam groove, a groove bottom is sloped such that a depth of the groove bottom continuously decreases from one end to the other end of the drive cam groove. The drive cam groove has: a first drive cam groove that is formed such that a distance between a center of a drive cam and the groove bottom at the first drive cam groove continuously changes from the one end toward the other end of the drive cam groove; and a second drive cam groove that is formed such that a distance between the center of the drive cam and the groove bottom at the second drive cam groove is constant from the first drive cam groove to the other end of the drive cam groove.