An auxiliary lock mechanism that locks a lock member (6) at a steering lock position is provided. In the auxiliary lock member, a slide plate (91) thereof engages with an auxiliary engagement portion (95) of the lock member (6) located at the steering lock position in such a manner that engagement between a holding member (21) and a holding/receiving portion (93) thereof is disengaged. On a more rear end side of the lock member (6) than the auxiliary engagement portion (95), a fragile portion (74) is provided, in which a load is set smaller than a load tolerable by the slide plate (91). In a case where a load of a set value or more is applied to the lock member (6), the lock member (6) is torn at the fragile portion (74).