PURPOSE: To enable a midway cut end section of the primary side cable to be surely connected to the terminal in any position of a terminal block by taking up a lead-in space of a cable wide enough to bend and lead in the primary side cable.

CONSTITUTION: A partition wall 25 is installed inside a casing main body 10 so that the open side of a through hole 16 may be narrow and the other side wide. The narrow side is for a space 26 for disconnection, while the wide side is for a space 27 in common use for setting terminal blocks and for leading in the cable. Inside the wide space 27 terminal blocks 28 are set towards the partition wall 25. To these terminal blocks 28 the primary side terminals 28a... and the secondary side terminals 28b... are arranged. Service entrances 29 and 29 are opened face to face each other on the long sidewalls 13 and 13 so as to lead in the primary side cable 30 through service entrances 13 and 13. The side space of terminal blocks 28 in the space 37 is applied to the space 27a for the exclusive use of leading in the cable.