PURPOSE: To inspect pinholes, sealing defects, etc. nondestructively and non-contaminatively by detecting the slight light transmitting through defective parts such as pinholes and sealing defects.

CONSTITUTION: A mouth part 12 of a paper vessel 1 is sealed with a sealing plate 13 of silicone rubber or the like, and a light source 14 is provided on the outer side of a bottom part 9. A slight light detector 15 is provided on the outside of the plate 13, and a photoelectric element 16 is placed within the dark visual field in the vessel 1 through the plate 13. The light source 14 is provided on the outer side of the part 9 because defective parts such as pinholes and sealing defects are liable to be produced in the joining sealed part between the side wall part 8 of the vessel 1 and the plate 9. The slight light leaking through defective parts such as pinholes and sealing defects is converted to electric current and if the value is below a certain value, it is judged as non-defective.