PURPOSE: To allow a steady burning of multiple briquets for a time period required for keeping warm and curing concrete after it is laid fresh by providing stays which are removably inserted through the wall of a hollow cylinder and, when inserted, can support the bottom of a briquet.

CONSTITUTION: Stays 7 are made of metal plate etc. and are slidably inserted to a hole 2d drilled in the side wall of an inner cylinder 2 and a hole 3c drilled in the side wall of an outer cylinder 3 for manual operation into and out of the cylinder in the horizontal direction. There are multiple (3 or more) stays 7 evenly arranged normally in the circumferential direction of the inner cylinder 2 and the outer cylinder 3 to support the bottom of a briquet 4 when inserted. After setting a lower briquet 5 in place, the stays 7 are pushed in, and an upper briquet 4 is placed and supported by the stays 7. In this arrangement, there is a gap 8 between the bottom of the upper briquet 4 and the top of the lower briquet 5. At the time of transfer of burning from the upper briquet 4 to the lower briquet 5, the gap 8 acts to supply air needed for burning, allowing a steady transfer of burning to the lower briquet 5.