PURPOSE: To ensure the stable retaining of a voltage transformer, and to improve a heat radiation efficiency by fitting a heat-conductive resilient member around the upper end of the voltage transformer and retaining the heat- conductive resilient member while it is pressed from its upper direction with a metal retaining fixture.

CONSTITUTION: A cap-shaped heat-conductive resilient member 18 is fitted around the outer periphery of an upper end 15b of a voltage transformer 15. The resilient member 18 is fixed while being pressed from its upper and lateral directions by means of a retaining metal fixture 17. This increases the degree of adhesion between the fixture 17 and the resilient member 18 and also between the resilient member 18 and the transformer 15 to a greater extent. Thereby, it becomes possible to adhere the resilient member 18 to a large area of both a top panel 15c and an exterior peripheral surface 15d of the upper end 15b of the voltage transformer 15. Accordingly, it is possible to ensure a large contact area of the fixture 17 relative to the transformer 15, which in turn causes heat generated by the transformer 15 to be efficiently dissipated and also a heat radiation efficiency of the transformer 15 to be improved.