To provide a manufacturing method that is superior in a mass-production property and can manufacture a seamless belt primarily comprising a thermoplastic resin.

The manufacturing method includes the steps of: extruding a molten body into a space between first and second cylinders from a discharge port of an annular die; clamping the molten body with the first and second cylinders; forming a tubular layer by relatively moving the first and second cylinders and the annular die in an axial direction; filling a gas into a space that is defined at least the tubular layer and an inner wall of the first or second cylinder; solidifying the tubular layer; bringing an annular cooling mold into contact with the molten body extending from the discharge port to the tubular layer by moving the annular cooling mold toward the annular die; sealing the discharge port by cooling the molten body with the annular cooling mold; detaching the tubular layer; and clamping the molten body that is solidified by cooling, with the first and second cylinders.