A security device for a vehicle having doors that can be locked and unlocked by using a mechanical key or an electronic portable unit, which transmits a signal of door unlock instruction. When the doors are unlocked, a monitoring state is reset and an alarm is unset, and it is checked if the doors are unlocked by using the mechanical key or by using the signal transmitted from the portable unit. If it is determined that the doors are unlocked by using the mechanical key, the user is informed of this fact.