28 to the connecting piece 27 c one flange 27 a constituting the torch holder 27 is formed perpendicularly to the connecting piece 27 c and the torch holder 27 is mounted to the supporting arm 25 b provided in the turning member 25 via the flange 27 a a structure of mounting the flange 27 a to the supporting arm 25 b is not particularly limited and the flange 27 a can be mounted to the supporting arm 25 b with use of a not illustrated bolt also the supporting arm 25 b can be formed in a half nut shape while the flange 27 a can be provided with a shaft member and the flange 27 a can be mounted to the supporting arm 25 b by fitting the shaft member into the half nut in the present embodiment the torch holder 17 is mounted to the supporting arm 25 b with use of a not illustrated bolt thus a hole 27 f through which the bolt passes is formed at a predetermined position of the flange 27 a although the circular hole 27 f is formed in the flange 27 a in the example illustrated in fig4 a linear elongated hole or an arc like elongated hole.