To provide a transformer with a tap changer for preventing gas leakage and the intrusion of rainwater at a box penetration part and easily installed.

The transformer includes: a winding 15 with a plurality of taps 21; an insulation medium 11 for insulating the winding 15; the tap changer 1 for selecting connection to the taps 21; an indication unit 7 for indicating a selected position of the tap changer 1; a main body box 3 for accommodating the winding 15, the insulation medium 11, and the tap changer 1; and a drive shaft 2 penetrated through the main body box 3 by means of the box penetration part 4 formed to an upper part of the main body box 3 to switch the tap changer 1. The transformer is furthermore provided with a drive unit box 23 fluid-tightly fitted to the main body box 3 at the upper part of the main body box 3 and arranged to cover the box penetration part 4 and the upper end of the drive shaft 2, and a drive unit 5 arranged in the drive unit box 23 and including a power source 20 to drive the drive shaft 2.

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