PURPOSE: To provide an inexpensive heat-resistant block, excellent in heat resistance, mass productivity, etc., and suitable for building furnaces by mixing volcanic ash with chamotte grains, sand, portland cement, refractory mortar and water and forming the resultant mixture.

CONSTITUTION: Volcanic ash is mixed with chamotte grains, sand, portland cement, refractory mortar and water. The resultant mixture is then formed into a shape having a rectangular parallelepiped form as a whole and a groove 2 from the side to the top surface thereof to produce a heat-resistant block 1. Thereby, since the volcanic ash, chamotte grains and refractory mortar are mixed in the resultant block 1, fire resistance is excellent. Since the block 1 forms no void and the shape is provided with the groove 2 formed from the side to the top surface thereof, impact resistance is excellent and the compressive strength for the net cross-sectional area is about 3 times that of normal concrete blocks. The block is preferably used for building refuse incineration furnaces, etc.