PURPOSE: To reduce the occupying area in a transformer tank by forming tripple drive shafts for a preceding switch, a trailing switch and polarity switch forming a tap selector coaxially as tripple drive shaft and reducing the number of insulating posts.

CONSTITUTION: A preceding switch F, a trailing switch G and a polarity switch H are provided under a gear box 21 normally in a tap selector E. Drive shafts 22, 23 and 24 for driving the switches are coaxially formed as a tripple drive shaft I. The shafts are smoothly rotated using bearings in such a manner that the length of the shaft is formed longer at the inner shaft, and the outermost shaft 23 is used as a switch G, an intermediate shaft 22 is used as a switch F, and the innermost shaft 24 is used as a switch H. Then, they are secured by insulating posts 29 mounted in the box 21, a plurality of insulating posts 28, 28aW28d having stationaly contactors 31, 30, 34 making contact with a movable contactor and a current collecting ring provided at the respective switches are provided around the shaft I.