PURPOSE: To eliminate a distortion by induction heating of an apparatus and lengthen its life by forming both side walls and a bottom wall into an integral body by means of a castable refractory material and by providing connecting plates maintaining and reinforcing the bottom at spaces each by an even number of times as long as the pole pitch of an inductor and at a right angle to the lengthwise direction of a melt passage.

CONSTITUTION: A melt passage 1 is formed into an integral body by means of a castable refractory material and its both sides are maintained by both side plates 2a, 2b of SS material. An upper trough is fitted at both ends with connecting flanges 5a, 5b for every given length. A non-magnetic connecting plate 3 is reinforced by connection of the lower edges of both side plates 2a, 2b by means of a plurality of narrow non-magnetic connecting plates 3 at right angles to the lengthwise direction of the melt passage 1. A space between said non-magnetic connecting plates 3 is an even number of times as long as the pole pitch of an inductor. Therefore, shifting magnetic fluxes passing through the non-magnetic connecting plates 3 offset each other apparently to enable elimination of a distortion by induction heating. Also, the melt passage 1 has a long life, because it is formed into an integral body and has no joint surface to generate no difference in melt.