A belt (3) of a continuously variable transmission comprises a pair of endless rings (2a) and plural elements (1) held between the pair of endless rings (2a). The endless rings (2) are formed of plural laminated thin plates (2a). A prestress in a compression direction is applied beforehand to the surfaces of the thin plates (2a) by a nitriding treatment. In this process, a defect part (2b) is formed as a hollow in the thin plate (2a) due to scatter in the nitriding treatment. This invention measures the depth of the defect part (2b), and if the measured depth exceeds a predetermined tolerance value, it is determined that the prestress is insufficient. Due to this determination method, a highly precise inspection of the thin plates (2a) forming the endless rings (2) can be performed in a short time.