A communication system includes: a vehicle-mounted communication device and a portable device. The communication device includes: a vehicle transmission unit which transmits a plurality of first signals to the portable device, and a vehicle reception unit which receives a second signal transmitted from the portable device. The portable device includes: a portable device reception unit which receives the first signals, a portable device controller which detects signal intensities of the received first signals, and which compares the signal intensities of the first signals, and a portable device transmission unit which transmits the second signal according to control of the portable device controller. The portable device controller sets an output value of the second signal to a predetermined value when a difference between the compared signal intensities of the first signals is equal to or less than a threshold.