Abstract: A cutter for a styrene board has an operation piece and a pair of cutter edges. The operation piece has at its forward end a flat surface coming into surface contact with the upper surface of the styrene board. The pair of cutter edges is attached to the operation piece so as to project in a mountain shape from the forward end of the operation piece. The lower end tip is formed at blades sections of inclined surfaces of the pair of cutter edges, and the tip is used to cut, in substantially a V-shape, that portion of the styrene board which excludes a sheet on the lower surface of the styrene board. Even an unskilled person can finely cut the styrene board in a V-shape, leaving the sheet on the lower surface of the board uncut.