(57) An on-load tap changer comprises a switch 18 such as a diverter switch or a selector switch including an current interrupting unit 20, a vessel 16 containing the switch together with an electrically insulating oil, a closed oil passage loop 43 including an oil conduit 38 having an inlet opening at the bottom portion of the vessel, the conduit extending through the upper portion of the vessel, passing through a level lower than the vessel, and connecting to the top portion of the vessel. The conduit has formed therein a small hole 44 at a position higher than the arc interrupting unit. The diameter of the small hole is sufficiently smaller than the inner diameter of the conduit to permit most of the insulating oil supplied into the oil filter unit 36 to be supplied from the inlet opening of the conduit at the bottom portion of the vessel rather than from the upper portion of the vessel.