dc converter and 12 a load as shown in fig1 and 2 the embodiment comprises the dc dc converter 11 and the power supply circuit 18 for supplying power to the dc dc converter 11 the power supply circuit 18 includes a capacitor 9 connected in parallel with input terminals e 0 and e 1 of the dc dc converter 11 the dc dc converter 11 includes a signal input terminal s in for receiving a standby completion signal i e a signal representing completion of charging of the capacitor 9 more specifically the power supply circuit 18 as a characteristic feature of the present invention comprises the first resistor 1 having one terminal connected to the positive potential terminal the positive side of the dc power supply 10 of the power supply circuit 18 to which one electrode of the capacitor 9 is connected a zener diode 6 having the cathode electrode connected to the other terminal of the resistor 1 and the anode electrode connected to the negative potential terminal of the power supply circuit 18 the load side of the dc power supply 10 a photocoupler having the anode electrode of the primary element 7 connected to the other terminal of the first resistor 1 the output electrode of a secondary element connected to the signal output terminal and the input electrode of the secondary element 8 connected to the other electrode of the capacitor 9 the mosfet 5 having the control electrode connected to the other terminal of the resistor 1 the input electrode connected to the load side terminal of the circuit 18 and the output electrode connected to the.