A hydrogen gas sensor capable of accurately measuring hydrogen concentration of a measurement gas atmosphere in the presence of a variety of interfering gasses such as H2O and CO. A transformer having four windings at a high-voltage side, the windings having the number of turns equal to one another, and four inductors are provided in correspondence with four switching circuits. When this occurs, an auxiliary pump is placed into operation to continue to supply the bearings with the lubricant until such time as the bearings have sufficiently cooled, at which point the auxiliary pump is shut down.