To increase a chance that a portable machine 10 receives a request signal, which is transmitted from a transmitter 20, in a detection area 50.

The transmitter 20 is provided on the side of a vehicle, and the request signal is transmitted according to a control signal from a computer 40. The control signal from the computer 40 includes instructions to change a transmission interval between the request signals, and instructions to change an output level of the request signal. The transmitter 20 transmits the request signal at the transmission interval and the output level, based on the instructions of the control signal. This can increase the chance that the portable machine 10 receives the request signal, which is transmitted from the transmitter 20, in the detection area 50.