To easily diagnose whether an on-load tap changing device is abnormal with high precision by seizing operation abnormality of an energy accumulation unit and a breaking unit provided to a changeover opening/closing device.

The on-load tap changing device includes the changeover opening/closing device 3 having the energy accumulation unit provided with a changeover crank forming a collision portion and performing rotary motion and a crank buffer device forming a buffer portion stopping the rotary motion of the changeover crank, and the breaking unit driven by the energy accumulation unit 21; a detecting means 8 of detecting a vibration signal generated when the collision unit of the changeover crank and the buffer portion of the crank buffer device collide against each other to stop the rotary motion of the changeover crank; and an abnormality diagnostic device 13 having a signal analyzing means 11 of analyzing the obtained vibration signal, and an abnormality determining means 12 of judging abnormality on the basis of the analysis result.