A signal transmission circuit provided with a control panel or a terminal device of a fire/security protection system is arranged so as to transmit a signal for the control panel or terminal device to a signal line. The signal transmission circuit includes: a signal transmission which is enabled when transmitting a signal under the control of a signal processing portion; a signal detector for detecting the signal transmitted from the signal transmitter to the signal line; a timer which is set when the signal is transmitted and is reset by the detection output from the signal detector, and a signal transmission controller for turning off the signal transmitter when the time count output of the timer exists during a predetermined time interval.