PURPOSE: To improve accuracy in quality inspection by imparting shock to the surface of a product, detecting the contact time between a striking tool and the product at the shock, and obtaining the internal property of the product based on the length of the contact time.

CONSTITUTION: The front end of a product 2 which is guided on a conveyer 1 is detected with a position detecting sensor 15. Then, the conveyer 1 is stopped, and the product 2 is located at a position directly beneath a striking tool 7 of a striking device 5. Then, an electromagnet 10 of the device 5 is instantaneously conducted, and attracting force is made to act on the striking tool 7. The striking tool 7 strikes the product 2 through a contact piece 5a. At this time, reaction is detected with a force sensor 6. The contact time, i.e. the time wherein the reaction occurs, is obtained based on the detected value. The quality of the product 2 is judged by the length of this time. In this inspecting method, when the contact time of the product 2 is longer than the specified time, the product is soft, and the occurrence of cracks, inner blisters, holes and the like is estimated.