PURPOSE: To simplify an inspection operation of tapping operation for absence of the operation by administering to a tap volatile lubricating oil and non- volatile coloring agent for performing a tapping operation to a drilled hole on a sheet metal part.

CONSTITUTION: In case when a thread-cutting operation is performed with a tap 3 on a drilled hole 1 of a part, such as a supporting metallic piece, this operation is performed by dropping on to the tap 3 operating oil 4: a mixture of volatile lubricating oil and non-volatile coloring agent. After the operation, the drilled hole 1 receives a threaded portion 5 and on a periphery zone the non- volatile coloring agent stays allowing visual confirmation that the required tapping operation has duly been conducted. Thus, an inspection for preventing infiltration of unoperated work can be conducted in a simple manner.