PURPOSE: To enable the measurement of the impedance of a module circuit on- line by providing an impedance circuit between supply source lines so that it is connected to the terminals thereof according to the operations of switching circuits, and by measuring the voltage of this impedance circuit.

CONSTITUTION: When a source voltage V between supply source lines 12 and 13 is supplied, a measured voltage Vi generated at signal terminals 14 and 15 is varied according to the operations of switching circuits S1 and S2. The impedance, i.e. resistances R1 and R2, of the signal terminal 14 of a module circuit 11 is determined by resistances R3 and R4 set beforehand and the measured voltage Vi varying in accordance with the operations of the switching circuits S1 and S2. Thus, the impedance of the signal terminal of the module circuit can be measured on-line by a simple circuit constitution and at the time of operation of the module circuit.