The present invention was achieved in order to provide an antitheft device, which can prevent unauthorized intrusion into a car even if an engine is started by remote control, to be mounted on a vehicle having a remote engine starting system which starts the engine based on a command from a remote control transmitter, comprising a controller to control the state of door windows so as to prohibit at least the opening out of the opening and closing of the door windows when the engine is started by the remote engine starting system.