PURPOSE: To provide a simple method for improving durability of hardened concrete by preventing a substance to deteriorate hardened concrete from penetrating through the surface of the concrete.

CONSTITUTION: The surface of hardened concrete is treated with an aqueous solution of a water-soluble inorganic salt having moisture absorption. Preferably, the surface of hardened concrete is coated with a cement paste, mortar or concrete containing the inorganic salt to improve durability of the hardened concrete.