PURPOSE: To obtain stable operation in any installing state by incorporating a switch changed over automatically in the installing state of a magnetic disc device into an offset circuit so as to prevent mis-application of switch or leaving it unswitched.

CONSTITUTION: When contacts 1, 2 of an automatic switch SW1 are connected, an input of an inverter 5 goes to an L level and the output goes to an H level and a switch SW2 is interrupted, while an output of a gate 6 goes to an L level, a switch SW3 is conducted and an output voltage of a circuit 3 is a low voltage via resistors R3, R4 and the SW3. When contacts 1, 3 are connected, both inputs of inverters 5, 7 go to an H level, an L level is given to two inputs of the gate 6, an H level is fed to a control input of the switch SW3, which is interrupted and the output voltage of the circuit 3 is zero. When contacts 1, 4 are connected, the inputs of the inverters 5, 7 go respectiely to H, L levels, the switches SW2, SW3 are both conducted and the output of the circuit 3 reaches a high offset voltage via the switches SW2, SW3 and the resistor R4.