To prevent the trouble due to bubbles when a thin film is formed by rapidly and surely detecting the bubbles in a coating soln.

The coating soln. is supplied to a nozzle 1 through a coating soln. feed pipe provided with an outside tank, a quantitative discharge and suction unit, a pintch valve, etc., and the coating soln. is supplied to the clearance between the surface SF of a substrate S to be coated and the nozzle 1 to form a soln. reservoir. Under such conditions, the nozzle 1 is moved along the substrate S, and the substrate is coated with the coating soln. by this coater. Besides, the upper part 131 of the main body 13 of the nozzle 1 is formed with a transparent body such as glass, a part of the pinch valve is also formed with the transparent body, and the state of the coating soln. in the nozzle 1 and pinch valve is visually confirmed.