In general, the step-like unevenness due to the fluctuation in the sheet-shaped molten resin in the vicinity of the surface of the cooling support remains as it is as thickness unevenness without undergoing leveling because the sheet-shaped molten resin is cast on the cooling support immediately after generation of the fluctuation to be solidified by cooling. However, according to an aspect of the present invention, the vibration of the sheet-shaped molten resin in the vicinity of the surface of the cooling support is specified to be 10 dB or less, and accordingly, the fluctuation of the sheet-shaped molten resin in the vicinity of the surface of the cooling support is suppressed and the sheet-shaped molten resin sufficiently undergoes leveling on the cooling support. Consequently, the generation of the continuous and periodic step-like thickness unevenness along the lengthwise direction of the sheet-shaped molten resin can be prevented.