A method for inspecting sealing defects of a container using infrared light. Infrared light in a wavelength range of 1450 nm±20 nm is irradiated from a phototransmitter onto the sealed portion of a sample container Infrared light reflected from or transmitted through the sealed portion of the sample container is received by a photoreceiver, and transmitted to a photodetector through an optical fiber. The infrared light in a same wavelength range as the case of inspecting the sample container is irradiated onto the sealed portion of a container to be inspected. The infrared light reflected from or transmitted through the sealed portion of the inspected container is converted into the analog voltage value and transmitted to a controller. If the infrared light is reduced to be smaller than a threshold set on the basis of the sample container, the sealed portion of the inspected container is judged as a sealing defect.