A communication device system includes a communication device (1) and a reception power monitoring device (2). The communication device (1) includes a reception processing unit (10) that outputs a signal intensity voltage (Vin) in accordance with a signal level of a received signal, and a monitoring terminal signal generator circuit (30) that generates a monitoring terminal signal (Vout) by current amplification of a level shift voltage generated by shifting a level of the signal intensity voltage (Vin) by an offset voltage (Voff) corresponding to a voltage value of a power supply voltage of an external device. The reception power monitoring device (2) includes a constant voltage generator circuit that outputs a constant voltage from the monitoring terminal signal (Vout), and a voltage converter circuit that operates based on the constant voltage and outputs a display level indication signal reflecting a voltage level of the monitoring terminal signal (Vout).