PURPOSE: To obtain a measuring method wherein the degree of the deterioration of a sheet of insulating paper used in the electric apparatus can be grasped accurately by a method wherein the measuring method is provided with a deterioration-causing gas measuring tank provided with the following which are respectively specific: the sheet of insulating paper and an insulating oil; a connection means; a heating means; and the sampling means of a sample for analysis use.

CONSTITUTION: The measuring method is provided with an oil-filled electric apparatus main body 1 in which an insulating oil is housed at the inside and which uses a sheet of insulating paper in order to insulate a winding and with a deterioration-causing gas measuring tank 2 which is provided with the following: a sheet of insulating paper 5 which is the same as the sheet of insulating paper and an insulating oil 11; connection means 3, 4 which are connected to the main body 1 so as to circulate the insulating oil by being connected to the main body 1; a heating means 6 which heats the insulating oil 11 at the inside up to a prescribed temperature; and the sampling means 10 of a sample for analysis use. For example, an insulating oil is circulated in a tank 2 by being connected to a main body 1 and the sample for analysis use of a first insulating oil is obtained from the tank 2. Then, an insulating oil 11 inside the tank 2 is heated up to a prescribed temperature, and the sample for analysis use of a second insulating oil is obtained from the tank 2.