PURPOSE: To make the surface of a panel smooth even if the panel is too rigid by forming a thick and dense layer adjacent to the surface of the panel, by a method wherein a peripheral speed of a round steel bar whose circumferential surface abuts against the surface of a lightweight cellular concrete panel is turned at the faster peripheral speed than a relative moving speed of the round steel bar and lightweight panel.

CONSTITUTION: A speed of a lightweight cellular concrete panel 2 moved by a moving mechanism 15 causing the panel 2 to perform reciprocation in the direction meeting at right angles with round steel bars 3, 4 is constituted lower than a peripheral speed of the round steel bars 3, 4. Concrete slurry in which cells are contained is placed within a concrete shutting 1 and operation is performed when the surface of the panel 2 becomes at least predetermined hardness, the cells close to the surface of the panel 2 are smashed and the surface of the panel 2 becomes so to speak, smooth like mirror finish. The surface of the panel 2 is rubbed under a state where great pressing force of a vertical direction is applied to a place close to the surface of the panel 2 and a thick and dense layer can be formed on the surface of the panel 2. Moreover, since the great pressing force is applied to the place close to the surface of the panel 2 by the round steel bars 3, 4, surface processing of the panel 2 can be performed even in the case where the concrete cures a little too hard.