PURPOSE: To shorten the heating time and to miniaturize the apparatus by supplying the air or gaseous oxygen into core sand and burning the sand, in the case of removing the core sand from a casting by burning the sand in a heating furnace.

CONSTITUTION: The casting W is charged into the heating furnace 1 lining refractory 3 in the inside of the body 2 and heated to a prescribed temp. A fan for uniformizing the temp. in the furnace is arranged at the bottom part of the heating furnace 1. In the opening part of the position using a core of the casting W, nozzles N connected with a header H are arranged to the faced position, and in the header H, compressed air or gaseous oxygen is supplied. In such a way, since the core sand is burnt by supplying the air or the gaseous oxygen, the heating time can be shortened to about 1/3 of the conventional method and combustion efficiency is improved.