PURPOSE: To make it possible to decide easily upon the degree of deterioration without taking time for sample collection by arranging the light emitting terminal and photo detection terminal of optical fibers at an interval in a transparent insulating material to be measured so that light axes will agree each other.

CONSTITUTION: In joint box 1, the light emitting terminal of optical fiber 6 with its one terminal connected to infrared light source 5 and the photo detection terminal of optical fiber 6' with its terminal connected to infrared analyzer 7 are exposed with light axes agreeing each other and sensor 9 molded by thermal hardening resin 8 is installed in insulating oil 4. Infrared rays, transferred to infrared analyzer 7, have parts of wavelength, equivalent to characteristic absorption of resolving product damped while passing through insulating oil 4, so that the kind and amount of the resolving product can be discriminated by measuring its wavelength and damping amount by infrared analyzer 7.