Abstract: A power saving device for a cable-information receiving/transmitting apparatus, for example, a facsimile machine or a computer. The device allows no AC power to be supplied to the apparatus during the stand-by time thereof. When ring signals arrives, or when the user picks up the handpiece of a telephone set attached to the apparatus, the Ac power supply is started. When the reception or transmission of information is over, the Ac supply is cut off after a predetermined time, thus allowing the after-reception work or after-transmission work to be finished. This device mainly comprises a relay (1) [or a SCR (1a)], a transistor (T), an optical coupler (3) [preferably consisting of a LED (31) and a CdS photoresistor (32)], and a tapping/rectifying circuit (2). It can totally eliminate the undeserved waste due to the consumption during the stand-by time.