To provide a mixture contact preventing plate without additionally providing a support structure and to improve a transformer in corona discharge- resistant properties in a withstand voltage test between a mixture contact preventing plate and a low-voltage coil.

A hollow insulator 4 is provided around a low-voltage coil 2, and a mixture contact preventing plate 7 is provided around the hollow insulator 4. The hollow insulator 4 is formed of a large number of rectangular glass chopped strands 5, where the chopped strands 5 are bonded together with insulating adhesive agents 6 so as to produce a large number of voids among them, and the total volume of the voids amounts to nine times or above as large as that of the glass chopped glass strands themselves.