To form a coating film having an even thickness and sufficient waterproofing performance to a porous material having an upper surface and both sides.

The method for coating an ALC plate 1 includes a step of preparing the ALC plate 1 on a surface of which an uneven pattern 2 is formed and spraying a coating material onto the upper surface 1a and both sides 1b, 1b of the ALC plate, a step of performing a first air blow onto the upper surface 1a to pour the coating material on the upper surface 1a of the ALC plate 1 onto which the coating material has been sprayed, to both sides 1b, 1b, and a step of performing a second air blow on both sides 1b, 1b after pouring the coating material from the upper surface 1a by the first air blow to both sides 1b, 1b.