PURPOSE: To prevent such that band forms turn, and intend to stabilize the device electrically, by inserting relay member which is composed of insulating material in the middle part of steel band, and winding the iron core.

CONSTITUTION: Two pieces of steel bands 3, 4, a relay member 5 in which one end of each band is connected, connecting members 7, 8 in which each another end is connected, and band fastening tool by which interval of connecting members can be adjusted are provided, and any one of relay member or connecting members is formed by the member which has electrical insulating property, for example, glass fiber or plastic. When this is wound on the iron core, turn is not formed by steel band, and fastening force which is stable electrically and is enough can be obtained.