To improve the operation rate of a transformer by detecting vacuum leakage of a vacuum switch tube, in a on-load tap changer using the vacuum switch tube in a running state of the transformer.

This on-load tap changer comprises a decomposed gas detecting means 31 for detecting the components and the amount of decomposed gases in an insulating medium in a housing oven 24, and detects vacuum leakage in the vacuum switch tube, based on the types and the amount of the detected decomposed gases. By adopting such a configuration, vacuum leakage in the vacuum switch tube 23a can be detected, without stopping the operation of the transformer, and the vacuum switch tube 23a having vacuum leakage which may be switched over repeatedly and may cause serious accidents due to disconnection failures can be prevented.