PURPOSE: To provide a method for protecting a concrete building body, made applicable to a wide range of concrete building bodies by coating a concrete building body with a combination of a high-molecular hydrosol with an aqueous protective coating liquid containing an alkali silicate as protective layer-forming component to effect permeating the coating agent into the concrete.

CONSTITUTION: For this method for protecting a concrete, a coating agent is used, being composed of (A) as protective layer-forming component, an aqueous solution of silicate at a concentration of 30-800g/L [the silicate, usually called 'water-glass', is a silicate of general formula, M2O.nSiO2 (M is an alkali metal: (n) is 1-3) in the form of simple substance or mixture] and (B) as auxiliary, a high-molecular hydrosol at a concentration of 50-800g/L such as a vinyl resin emulsion, asphalt emulsion or SBR latex., at the weight ratio A/B of 100:(5-100) on a solid basis. For practical application, a concrete building body is coated with the coating agent or the components A and B simultaneously to effect permeating the coating agent into the concrete, thus the silicate convertd into an insoluble compound forms the protective layer ranging from the surface of the concrete building body to the inside thereof.