PURPOSE: To detect the diameter of a hole, which is excessively larger than a specified size, simply, accurately and efficiently, by providing a pressure chamber under a minutely perforated film, which is horizontally moved, providing a liquid chamber over the film, and detecting bubbles in the liquid chamber caused by gas, which has passed the defective part of the minutely perforated film.

CONSTITUTION: An air pressure chamber 11 is provided under a minutely perforated film A, which is moved horizontally. A liquid chamber 12, at a bottom of which a sintered metal plate 19 is formed, is provided over the film A through elastic bodies 18 and 22. An organic solvent 20 is injected into the liquid chamber 12 to a specified height, and specified gas pressure is applied to a uniform pressure chamber 13 and the air pressure chamber 11. When a defect larger than the specified diameter of a hole exists in the minutely perforated film A, which passes between the pressure chamber 11 and the liquid chamber 12, the pressure gas in the pressure chamber 11 goes to the upper side of the minutely perforated film A, passes through the sintered plate 19, and becomes bubbles, which rise up in the organic solvent 20. Therefore, the defect can be detected simply, accurately and efficiently.