To automatically confirm the existence/absence of all rings in the course of assembling a product by checking the existence/absence of a ring existing in a groove according to the quantity of air flowing through an opening part measured by an air pressure sensor for a test head.

When a test head 404 is lowered to be disposed on a groove 110 of a piston 100, a micro-controller checks the existence/absence of all rings 112, 114, 116, at least one of which exists in the groove 110 of the piston 100 according to the digital value of voltage indicating the quantity of a second air current flowing through an opening part 402. Further, the micro-controller can use a digital value of voltage indicating the quantity of a first air current flowing through the opening part 402 of the test head 404 at the time of checking the existence/absence of all rings 112, 114, 116, at least one of which exists in the groove 110.