PURPOSE: To detect whether the crashing amounts of the O ring is within an allowable size or not readily and accurately, by using a gage having steps which are equivalent to the specified maximum and minimum crashing amounts of the O ring.

CONSTITUTION: Errors are caused in the crashing amount ΔW of the O ring 1 due to the errors in the diameter and the depth of the groove of the O ring in the actual application. The maximum allowable range of the crashing amount is specified from the maximum ΔWmax to the minimum ΔWmin, and the conductive gage 8 with the steps 8a and 8a' having said magnitudes are prepared. The gage 8 and a flange 3 having the groove are electrically connected by electric wires 9, 9', and 9", a battery 10, and a lamp 11. By observing the lighting of the lamp, the check is performed to see whether the crashing amount is within the specified magnitude range of ΔWmaxWΔWmin.