PURPOSE: To eliminate the seasoning and hardening of paint at a knife edge part and uneven paint-application, by feeding a wide relatively slower-speed gas stream in from the rear periphery of a cup to be coated when the paint is diffused by the high-speed rotation of the cup.

CONSTITUTION: Compressed air is supplied through the hollow part of a rotary driving shaft to an air-spraying ring 39 in close contact with the rear periphery of a cup 1 to be coated, and sprayed to the periphery of the cup 1 to be coated through many small holes formed at the front surface part of the air-spraying ring 39 along its circumference. The sprayed shaving air stream expands its width until it reaches a knife edge part 9 at the front end of the cup 1 to be coated, so that the speed of the stream becomes slow and that drying the paint at the knife edge part 9 is not accelerated. Since the gas stream itself is widened although its speed becomes slow, the diffusion of the paint does not spread so much.