An objective of the present invention is to further reduce power consumption. A key telephone system is formed from a terminal and a primary device. The primary device comprises a transmission means for transmitting power conservation sustaining time data which is data which indicates power conservation sustaining time which is the time wherein a power conservation state which is a state wherein power consumption is further reduced is sustained together with an instruction for transition to the power conservation state to the terminal when a power conservation state transition time is reached which is a time for transition to the power conservation state. The terminal comprises: a communication means for carrying out a communication with the primary device; a timekeeping means for measuring the passage of a set time; and a control means for setting the power conservation sustaining time upon the timekeeping means and interrupting the communication means upon receipt of the power conservation sustaining time data and the instruction for transition to the power conservation state, and for activating the interrupted communication means when the timekeeping means has measured the passage of the power conservation sustaining time.