PURPOSE: To improve facilities of a constant-voltage output circuit by providing said output circuit with constant-current elements causing a constant current to flow independently of a voltage level of applied voltage.

CONSTITUTION: A DC input unit for programmable controller is constituted by connection with a photo-coupler PC of an input circuit 1-1 for taking a constant current from a DC voltage to be inputted from an external electrical equipment 2 via a contact K and an output circuit 1-2 for generating a constant DC voltage to be outputted to said controller on the basis of said constant current. In this case, a constant-current diode DCR having constant-current characteristics is connected between a +terminal T1 of the DC input unit 1, to which voltage is applied from the external electrical equipment 2, and a light emitting diode PD of the built-in photocoupler PC. As a result, the constant- voltage diode DCR causes a constant current to flow to the light emitting diode PD independently of a voltage level of the applied voltage and said diode PD to emit light, and gives an operating voltage to C-MOS-IC via a capacitance CC.