Patent ID: 7507462

Claim:
A plastic sheet comprising: a first surface; and a second surface that is not provided on a same plane as the first surface, wherein the first surface and the second surface are joined with each other on an imaginary line, each of a first straight line and a second straight line, which are parallel to and equally spaced from the imaginary line, has a plurality of cutout portions, a projection of each of the cutout portions provided on the first straight line, on the second straight line is placed between adjacent cutout portions provided on the second straight line, a projection of each of the cutout portions provided on the second straight line, on the first straight line is placed between adjacent cutout portions provided on the first straight line, and a first line segment connecting a second end opposite to a first end of a first cutout portion provided on the first straight line to a first end of a first cutout portion provided on the second straight line, a second line segment connecting a first end of a second cutout portion provided on the first straight line, which adjoins the second end of the first cutout portion provided on the first straight line, to a second end of the first cutout portion provided on the second straight line, and a third line segment connecting a second end of the second cutout portion provided on the first straight line to a first end of a second cutout portion of the second straight line, which adjoins the second end of the first cutout portion provided on the second straight line, are orthogonal to the imaginary line.