PURPOSE: To enhance reliability in the inspection of insulation by ascertaining the contact between an inspection probe and a terminal of a circuit to be inspected prior to the measurement of insulation resistance thereby preventing the erroneous decision due to noncontact of the inspection probe.

CONSTITUTION: Inspection probes 4a, 4b; 5a, 5b; 6a, 6b; are brought into contact, respectively, with terminals 1a, 1d; 2a, 2b; 3a, 3b of networks C1-C3 formed independently on a printed circuit, board A. After switches S1, S2 are turned to contacts T1, T4, a switch S3 is turned ON and if the resistance between the probes 4a, 4b is lower than a predetermined value, a decision is made 7 that the contact between the probes 4a, 4b and the terminals 1a, 1d is normal. Subsequently, the switches S1, S2 are switched sequentially to contacts T2, T5 and T3, T6 and the resistances between the probes 5a, 5b; 6a, 6b are measured thus ascertaining the contact. When the normality is ascertained for all contacts, the switches S1, S2 are turned to the contacts T1, T5 and the switch S4 is closed before the insulation resistance is measured between the networks C1, C2 Similarly, the insulation resistances are measured between the networks C1, C3 and between the networks C2, C3.