PURPOSE: To attain to enhance the operation reliability of oil-filled electric machinery by making it possible to diagnose the residual life of said electric machinery during operation, by mounting a temp. detecting means, an oxygen concn. detecting means, a memory means of detection output, a memory means for storing deterioration characteristics of an insulating material and an operation means for calculating residual life.

CONSTITUTION: Converter apparatuses 10A, 10B and data memory apparatuses 11A, 11B are operated at every definite period during the operation of a transformer 1 by the control of a central control part 17 while the outputs from a measuring instrument 8 for measuring dissolved oxygen in oil and a operation temp. measuring instrument 9 are extracted as electrical signals and successively stored in the respectively corresponding data memory apparatuses 11A, 11B. In this case, these memory data are read at every certain period and imparted to a comparing operating apparatus 12. Whereupon, the comparing operating apparatus 12 calculates an oxygen concn. change amount from said read and calculated data and further compares this oxygen concn. change amount on the basis of the data stored in an experimental data memory apparatus 13 and an oxygen transmission data memory apparatus 14 to calculate residual life due to the deterioration of an insulating material.