power to a power transformer 13 via a power switch 12 the secondary winding voltage of the transformer 13 is rectified by a full wave rectifier circuit 14 and is filtered by a filtering capacitor c 1 the output of the capacitor c 1 which is connected to ground at the positive potential side is supplied to a dc dc converter 15 the dc dc converter 15 varies the input voltage and its output is supplied to the cpu 1 i o port 4 printing section 5 display section 6 key input section 7 loudspeaker 8 and collector 24 between the ground and one end of the secondary winding of the transformer 13 there is connected a series circuit constituted by a diode 16 and a capacitor c 2 the capacitor c 2 has a much smaller capacitance than the capacitor c 1 the voltage built up at the connecting point of the diode 16 and capacitor c 2 is applied to the or circuit 309 via an inverter 310 and also to one input terminal of an and circuit 19 via inverters 17 and 18 the other input terminal of the and circuit 19 is supplied with the signal ce1 from the cpu 1 the output of the and circuit 19 is supplied as chip enabling signal ce1 to the memory circuit 2 further provided is a battery 20 for supplying power to the memory circuit 2 time keeping memory circuit 3 and time keeping circuit 9 in case the power source section 10 is turned off or the power supply is cut by accident the time keeping memory 301 is constituted by a ram which has such a memory location as illustrated in fig3 as.