When the molded product is heated to a certain temperature, the aluminum dihydrogen phosphate becomes aluminum dihydrogen pyrophosphate and aluminum metaphosphate, and the polymerization reaction occurs accordingly, which is formed at a temperature of 400 to 500 ° C or higher. The aluminum phosphate polymer "Al[PO3]3"n, with the increase of the degree of polymerization, makes the amorphous refractory material form a strong bond, and the strength is obtained. The aluminum metaphosphate is also polymerized while also decomposing part of P2O5 and refractory material. The formation of AlPO4 in the Al2O3 reaction in the same process is also advantageous for the improvement of the strength of the amorphous refractory.