To obtain a coating agent capable of forming excellent glaze layer capable of providing improved flatness and durability to a refractory by a low temperature application by formulating a specific proportion of a thickening agent to an aqueous solution of an alkali silicate to regulate the viscosity of the aqueous solution at a specified temperature so as to be a specific value.

This coating agent for a high temperature refractory, regulated so that the viscosity of the aqueous solution at 20°C may be 800-10,000 cp is produced by formulating 0.2-10 wt.% thickening agent with an aqueous solution of an alkali silicate. An inorganic thickening agent such as sepiolite and bentonite, and an organic thickening agent such as a carboxymethylcellulose and methylcellulose are cited as the thickening agent. Sodium metasilicate, potassium metasilicate, sodium orthosilicate, potassium orthosilicate or the like is cited as the alkali silicate. An alkali borate (e.g. sodium metaborate) can be used together with the alkali silicate, and 5-15 wt.% B2O3 is preferably included in the glaze layer.