PURPOSE: To manufacture a plastic sheet of correct folding angle and good flatness after assembly by providing two rulings formed by recessing one face of the plastic sheet and one ruling formed by recessing the position corresponding to the middle between two rulings on the other face.

CONSTITUTION: A folding line 1 on a plastic sheet 2 is formed inside the thickness of the sheet 2, and a folding line is formed by two rulings 31 and 32 formed by recessing one face of the sheet 2 and one ruling 33 formed by recessing the other face on the position corresponding to the middle between rulings 31 and 32 on the other face. The folding line 1 can be finished thin by the arrangement, and as the folding stress applied on both faces of the sheet 2 can be dispersed and absorbed into three rulings 31-33, folding can be carried out easily and accurately. Also as the residual stress is small, flatness after assembly can be made good in the case of assembling into a doll case or a small box for food.