A constructional finished wallboard is formed of a mixture including 50 to 65 wt. % of dried shirasu, 30 to 40 wt. % of a plaster component, 3 to 10 wt. % of a clay component, 5 to 10 wt. % of shirasu balloons, 0.8 to 2 wt. % of a bond reinforcing component, 0.01 to 3 wt. % of a color pigment, and 1 to 5 wt. % of fibers for plaster. In preparing the wallboard, after preparing the mixture, an appropriate amount of water is added to the mixture to mix together, which is applied onto a substrate. Then, the surface of the mixture on the substrate is finished, followed by drying to thereby form the constructional finished board.