PURPOSE: To improve reliability by inputting a control signal from output inhibiting means to a logical circuit with an unknown logic in mounted logical circuits to inhibit the logical circuit from generating an output and testing the other logical circuits in a test.

CONSTITUTION: When a printed circuit board unit 1 to which a plurality of logical circuits are mounted is tested, by inputting a test/actual operation change over signal to a logical circuit 21-2 with an unknown circuit structure, for example, such as the LSI of an MPU, the output of the circuit 21-1 is inhibited. At this time, a test pattern is inputted to a logical circuit 21-1 with a circuit structure known in advance and the circuit 21-1 is tested at first. An output signal (a) from the circuit 21-1 is used an input signal to the logical circuits to be subject to a test to be performed later. The signal (a) skips over the circuit 21-2 by short-circuiting it by a switch 23 via probes 22 and the signal (a) is made an input signal (b) to another logical circuit 21-3 with a known circuit structure whereby the circuit 21-3 is tested next. The output signal (c) of the circuit 21-3 is made the output pattern of the unit 1 and compared 24 with an expected value to judge defective or normal.