To surely detect defective assembling by directly rotating the snap ring with a detecting shaft having a torque meter, and judging the assembling quality of the snap ring based on the rotational resistance, i.e., the size of torque.

A torque detecting head 4 is lowered and stopped at the position where a probe 40 stores a snap ring. A motor 46 is started, a torque detecting shaft 42 is excited downward while being rotated at a relatively low speed, the notched recess of the probe 40 is coupled with the outer peripheral projection of the snap ring and rotated, and the rotational torque is measured by a torque meter 41. The actual torque value is determined by a judgment section 5 based on the measured torque signal, and this torque value is compared with the prescribed torque value. When the compared result is the prescribed value or below, the assembling of the snap ring is judged as normal, and an OK lamp is lighted on a display section 6 together with the torque value.