PURPOSE: To obtain an easy folding work and good appearance, by forming a complex and recessed grooves at the jointing parts of the ruled lines and a plastic film with a half thickness of the plastic sheet.

CONSTITUTION: Folding ruled lines 4 made by prolonged lines are provided on a plastic sheet of a complex sheet 1 which is composed of the thick plastic sheet 2 and a plastic film 3 half as thin as the plastic sheet, bonded with the sheet and recessed grooves 6 are formed at the jointing parts of the ruled lines. In this way, the sheet 2 of the jointed parts 5 can be prevented from projecting to the film side 3 on bending and an assembly with a good appearance can be obtained. In particular, if the film 3 and the recessed groove are processed together, it is more effective. And further, the recessed grooves 6 of the jointing parts 5 are formed slender to make the recessed grooves 6 to be the folding centerline in the case of wide ruled lines 4. And hence, it can be always bent at the center position of the ruled line 4 and a good assembly with dimensional precision can be obtained.