PURPOSE: To reduce a size of the choke groove and facilitate a design of the door by a method wherein a dielectric means is arranged in the choke groove formed at the inner circumference of the door and then a transparent screen and a cover are arranged in sequence in the inner part of the door.

CONSTITUTION: When the door is assembled, a dielectric means 4 of polypropyrene etc. is stored in the choke groove 2 having a substantial "ko"(Japanese:katakana,2533)-shaped section arranged at the inner circumferential part of the door 1 made of thin magnetic metal plate, and the metallic transparent screen 5 is welded to the inner surface of the door 1. Then, a transparent cover 8 of polypropyrene resin is arranged at the inner side of the screen 5 and a transparent window 9 of polypropyrene resin is stored in the opening 3 of the door 1 so as to perform a dielectric heating. The dielectric means 4 is melted and partially flowed into a plurality of holes 10 made in the choke groove 2 so as to be fixed therein. In turn, the cover 8 is melted and fixed to the screen 5, and the transparent window 9 is also fixed to said opening 3 and the screen 5, respectively.