PURPOSE: To carry out sure and easy inspection of a defect inside a light-transmitting object without necessitating a liquid of an equal refractive index and in a noncontact manner by placing the focus for detection on the defect through the end face of the light-transmitting body.

CONSTITUTION: On the occasion of detecting a defect 10 inside a lighttransmitting object 2 of which the surface and rear are light-scattering faces 4, a laser light 7 is applied through the light-scattering face 4 or another end face from a light source 6. The laser light 7 becomes a scattered light source at a line 9 on the light-scattering face 4, and when the defect 10, e.g. a bubble, exists on the face, the light is reflected and scattered and forms a luminant spot. A lightsensing system and a photodetector, e.g. a camera 1, placing the focus on a plane being parallel to the end face 3 and containing the line 9 come to place the focus on the luminant spot of the defect 10 and detects it. By scanning the object of inspection in this way, inspection of the whole object is enabled. Thereby the defect 10 can be specified on a linearly defined section on which the laser light 7 hits, that is a scanning position, and further it can be detected three-dimensionally from the detected position of the luminant spot in the camera 1.