A control system includes a vehicular device equipped to a vehicle and a portable device carried by a user. The vehicular device transmits, at a first transmission time interval, an RF signal in synchronization with an RF clock signal after a transmission of an LF signal. The portable device counts a reception time interval of the RF signal and determines, based on the reception time interval, a second transmission time interval of a response signal, and transmits the response signal at the second transmission time interval. The vehicular device further determines whether a duration time from transmission of the LF signal or the RF signal to reception of the response signal is within an appropriate range set based on the RF clock signal, and permits a predetermined manipulation to the vehicle when the duration time is within the appropriate range.