To obtain a measuring method in which the residual quantity of a machining oil remaining on the surface of a metal product cleaned with a cleaner after a metalworking operation can be measured precisely.

In a method, the residual quantity of a machining oil remaining on the surface of a metal product cleaned with a cleaner after a metalworking operation is measured. At this time, the metal product is put into a cleaning tank containing an organic solvent which can dissolve the remaining machining oil, the remaining machining oil is dissolved and extracted, an extracted liquid which is obtained is filtered by a sheet of inorganic filter paper, and the residual quantity of the machining oil is determined by an oil-portion concentration meter or an infrared absorption spectrum analysis regarding an obtained filtrate.