PURPOSE: To attain connection or disconnection at a suitable time, by providing a switch control circuit and designing a device that a magnetic disc device discriminates and executes the control of connection or disconnection of control lines connected between the magnetic disc controller and a magnetic device.

CONSTITUTION: When switches 8 or 9 is connected, an output of an NOT circuit 38 goes to "0" and then, an outout of an NAND circuit 39 goes to "1" and an output of an NOT circuit 41 becomes to "0". Thus, an output of an NOT circuit 44 goes to "1" and an output (b) of an OR circuit 47 remains "1" so as to block the disconnection of switches 6 or 7. When switches 8 or 9 is disconnected, an output of the NAND circuit 39 goes to "0" and an output of the NOT circuit 41 goes to "1", then an output of an OR circuit 46 goes to "1" and the output of the NOT circuit 44 goes to "0". Since the output of an NAND circuit 40 is at "1", an output of an NOT circuit 45 goes to "0", an output (b) of the OR circuit 47 goes to "0" so as to attain the disconnection of switches 6 or 7.