A public telephone set includes a communication enabling unit, a signal detector, a timer, and a CPU. The communication enabling unit enables incoming communication upon formation of a DC loop based on an incoming call. The signal detector detects a protocol signal from a maintenance center or a service center via the office line. The timer is started upon formation of the DC loop. The CPU instructs to send out alarm information to the maintenance or the service center via the office line when the signal detector generates a detection output during an operation of the timer.