PURPOSE: To separate a high voltage section and a low voltage section in distance along with a large isolation voltage by providing a light emitting element at the high voltage section to emit light when it causes an overvoltage and a light receiving element at the low voltage section as connected together through an optical fiber.

CONSTITUTION: PD indicates a light emitting element, PT a light receiving element and F an optical fiber. It is assumed that as voltage between terminals (a) and (b) connected to a high voltage section is higher than the Zener voltage of a Zener diode ZD, a Zener current flows. At this point, the light emitting element PD emits light, which is received with the light receiving element PT through the optical fiber F. This allows a low voltage circuit DET to detects the generation of an overvoltage at the high voltage section. This also enables the separation of a high voltage section packaged on a printed board Pt1 and a low voltage section packaged on a printed board Pt2.