PURPOSE: To simply measure OFF characteristics of a semiconductor rectifier element unit connected in series or in parallel to constitute a power converter device, and judge whether the unit is good.

CONSTITUTION: An inverse voltage is impressed from a variable DC-stabilizing power source 21 to a to-be-inspected semiconductor rectifier element unit 1. A leaking current a predetermined time later is detected by a leaking current- detecting part and input to a settling circuit 23. The settling circuit 23 compares the current with a set value set beforehand from a measured initial value of the element unit 1 to judge whether the element unit 1 is good or not. An abnormality of a partial voltage resistance, a snubber circuit inside the element unit 1 are also judged at the same time.