A coated film forming method is capable of readily forming a coated film of a thermoplastic material on a coated region of an inner peripheral surface of a cylinder. A nozzle, through which a molten paste of a thermoplastic material kept molten is discharged, is disposed toward an inner peripheral surface of a cylinder. The nozzle is moved along a rotational central line of the cylinder while rotating the cylinder and discharging the molten paste from the nozzle. Centrifugal force acting on the cylinder being rotated leads to spreading of the molten paste applied to the inner peripheral surface of the cylinder. Viscosity of the molten paste, a rotational speed of the cylinder and a speed of movement of the nozzle are set so as to keep the molten paste discharged from the nozzle from being scattered to a region other than the coated region.