PURPOSE: To efficiently and inexpensively produce a case by employing a device capable of simply applying various color designs to a part of a plastic sheet.

CONSTITUTION: When a design is applied, heater blocks 14a-14d fitted with heater heads 16a-16b having design molds provided to the leading end parts thereof are used and a printing film 26 for the thermal transfer of ink is arranged between the heater heads 16a-16d and a plastic sheet 22 to be pressed by the heater heads 16a-16d to partially heat and press the surface of the plastic sheet 22. Whereupon, the pressed parts are melted to form depressions and ink is transferred to the depressions to form patterns becoming a design. When the design is changed, the design molds provided to the heads 16a-16d or the printing film 26 may be replaced. Therefore, when a multicolor printing film is used as the printing film 26, multicolor printing is enabled by one press process.