A wireless communication system for a vehicle has a portable device that transmits a wireless signal and a vehicle-mounted device that receives the wireless signal from the portable device. Each time a switch in the portable device is turned on, the portable device updates a portable device rolling code and transmits a wireless signal including the portable device rolling code. The vehicle-mounted device receives the wireless signal through a receiver and authenticates the portable device through comparison between the portable device rolling code included in the wireless signal and the vehicle rolling code. The vehicle-mounted device locks or unlocks the vehicle door if the portable device is authenticated. When the wireless signal is received through the receiver after the most recent locking or unlocking of the vehicle door has been carried out using the mechanical key, the vehicle-mounted device locks or unlocks the vehicle door if the switch is turned on for n (n is an integral number greater than or equal to 2) times or more.