PURPOSE: To obtain high perfomance as a heat insulating material for a molten metal by using a hollow granular body having a bulk specific gravity and grain size in a specific range for said material.

CONSTITUTION: A heat insulating material is constructed of a hollow granular body of which the shell is constituted of a heat-resistant inorg. material and which is calcined to ≤0.5 bulk specific gravity and ≤30mm and ≥3mm max. size. Such heat insulating material is used as a heat insulating material for the molten metal in a ladle lined with bricks consisting essentially of magnesium oxide, by which the erosion of the ladle bricks is prevented and the working efficiency of the ladle is improved without damage on the bricks for long time.