A security system, which is introduced into a vehicle or a building having one or more doors, includes a receiving circuit to receive a prescribed remote control signal, an unlocking control mechanism to make a locking mechanism or locking mechanisms to one or more doors be in the unlocked state when the prescribed remote control signal is received, a closing detecting mechanism to detect the closing of a door, and a locking control mechanism to make the unlocked locking mechanism or mechanisms be in the locked state, when the door that is opened after the door or doors are unlocked, and then the closing of the opened door is detected.