A key slot capable of accommodating an auxiliary key, which is a vehicle key other than a master key. A key lock mechanism locks or unlocks movement of the auxiliary key in the key slot, thereby allowing the auxiliary key to be taken out of the key slot in accordance with a lending condition of the auxiliary key. An invalid key return prevention unit permits the auxiliary key to be returned to the key slot in accordance with a return condition of the auxiliary key. If the return condition is met, the invalid key return prevention unit causes the key lock mechanism to lock the auxiliary key in the key slot. If the return condition is not met, the invalid key return prevention unit does not allow the auxiliary key to be returned to the key slot.