PURPOSE: To avoid the clogging of preheated air outflow unit by a method wherein the preheated air outflow unit is constituted of a multitude of short and small tubular units while the oblique section of the short and small tubular units is positioned at the side of a heat storage chamber.

CONSTITUTION: A multitude of short and small tubular units 1, having oblique section 2 on one end thereof, are bundled and connected so that the axes of the tubular units 1 become parallel to constitute the preheated air outflow section 4 of a heat storage chamber 3 while the side of the oblique section 2 is positioned at the side of the heat storage chamber 3. The oblique section 2 can be provided on both sides of the short and small tubular units 1. According to this method, heat storage balls 5 will never be fitted into the holes 6 of the short and small tubular units and, accordingly, the clogging of the preheated air outflow unit 14 can be prevented.