To provide an infiltrable coating agent which infiltrates into an irregular surface or gap such as a finely irregular surface and fine cleavage while expelling air from the gap, etc., and hardenable by the evaporation of a solvent.

The infiltrable coating agent is a liquid prepared by dissolving a synthetic resin polymer and a surfactant in a solvent, and the liquid infiltrates into an irregular surface or gaps such as a finely irregular surface and fine cleavage of an air-permeable material while expelling air from the gap, etc., and hardened by the evaporation of the solvent.