To provide a plate material improved in rigidity by providing an irregular part, which has a pattern of irregular part further high in the rigidity improving effect.

The irregular part 2 is constituted by combining a number of three types of areas, or a first area 21, a second area 22 and a third area 23 differed in projecting height in the thickness direction. Each area is dispersively arranged so that each area necessarily contacts with the other different two types of areas without being continued to the same type of area. The first area 21 is dispersively arranged in a regular manner in conformation to the center point of each hexagon constituting a virtual hexagonal lattice and the position of each grid point, the second area 22 is dispersively arranged so as to mutually connect the adjacent first areas 21, and the third area 23 is dispersively arranged in a position surrounded by the first area 21 and the second area 22.