detecting circuit a base emitter voltage vbe of a transistor increases according to an increase in a collector current from this viewpoint the current detecting circuit constituted by the transistor q 3 can be observed as functioning in the following manner a bias current an amount of which corresponds to the current of transistor q 1 which is detected by the transistor q 3 is supplied to the reference voltage supplying circuit 7 or 4 in the circuit shown in fig2 b by supplying such bias current the base emitter voltage vbe q 2 is equalized to the base emitter voltage vbe q 1 and the output voltage vout coincides with the zener voltage vz with high accuracy a constant voltage supplying circuit 11 as a second embodiment of the present invention is shown in fig3 in this embodiment a constant current supplying circuit 12 is additionally connected between the power source line 1 and a cathode of the zener diode d 2 other structures are the same as those of the first embodiment an amount of current i 2 of the constant current supplying circuit 12 is set to an amount that is sufficiently high to establish a stable zener voltage vz in the zener diode d 2 in this second embodiment even if the current i 1 from the current outputting circuit.