PURPOSE: To make reliable measurement possible without requiring complex operation before and after the measurement, by providing the printed wiring of a measured part to the circumference of a through hole where a probe penetrate.

CONSTITUTION: Printed wiring 22 is provided to printed-wiring plate 20 to make a connection between a power terminal and cold terminal 43 of high-tension winding 42 of fly-back transformer 4. In the middle of it, through hole 23 of a little bit thicker than the outer diameter of magnetic-core probe 18 wound with pickup coil 8' is made and printed wiring 22 is provided around the outer circumference of the through hole in a "Ω" shape. A detection pattern like this can be provided previously to any measurement point in a print-wiring process. In this way, reliable measurement can be made without requiring complex operation before and after the measurement.