PURPOSE: To test the invisible connection part of an electronic part without destructing the electronic part by applying different voltages to the electronic part of a mounting substrate for a short time and measuring respective current values.

CONSTITUTION: A voltage setting part 1 sets applied voltage and a voltage applying time and a voltage generating part 2 generates the voltage indicated by the voltage setting part 1. When the indicated voltage is applied, a current passes through a connection pattern from a measuring terminal 5 to flow to a measuring terminal 6 through a resistor. When the applied voltage is low, no current flows to an NPN transistor. The current flowing to the terminal 6 is measured by a current measuring part 4 and, when a predetermined current flows, the connection state of the output line of an OR gate can be confirmed. Next, when higher voltage is set in the same way to carry out measurement, a current flows in the same way but, since applied voltage is high, an NPN junction becomes a continuity state and a current is measured by the current measuring part 4 to make it possible to confirm the connection state of an input line. By this method, the invisible connection part of the electronic part can be tested without destructing the connection part.