To improve clamping operation of a band by providing a tap to an upper part clamp of a three-phase pole-mounted transformer or welding a hexagon nut to the backside of the upper part clamp so that a tap stage is fixed to the upper part clamp using a pipe and a screw after clamping a band.

A tap 8 is opened at an upper part clamp 2 only upper surface of a tank of a three-phase pole-mounted transformer, a pipe 7 is placed on the tap 8, and a tap stage 4 is placed over it so that the tap stage 4 and the pipe 7 are fixed to the upper part clamp 2 with a screw 6. After a hexagon nut 9 is welded (11) to the backside wherein a hole 12 is opened at the upper part clamp 2, the pipe 7 is placed over it, further the tap stage 4 is placed, which are fixed with the screw 6. Splitting a part for fixing the tap stage 4 makes an operation for clamping a band 3. Thus, the inside diameter of the tank is reduced while waste of an insulating oil is eliminated.