PURPOSE: To more surely diagnose abnormality by performing the gas analysis of an insulating medium in a changeover switch chamber and diagnosing the internal abnormality of a changeover switch from the change of the kind, the product quantity and the product ratio of a decomposition product.

CONSTITUTION: The kind and quantity of a decomposition product of an insulating oil 5, which is generated inside a changeover switch 2, are detected by a detector 10 and displayed by a display device. When either one of gases of CH4 and C2H4 mug the gases of H2, CH4, C2H6, C2H4 and C2H2 which are extracted from the insulating oil 5 is most generated, it is supposed that a loaded tap changeover switch in which the insulating medium is enclosed, has local abnormal overheat. Further, when a total generation quantity of decomposition gas exceeds 10000ppm it is also supposed that the internal abnormal overheat is brought about. Thereby, the diagnosis of abnormality of the loaded tap changeover switch in operation can be more surely performed at an early stage.