To grade a torsion spring to be measured, as to whether or not it satisfies required conditions by a method, wherein the torsion spring to be measured is wound and upwound and the relationship between a series of known winding positions and unwinding positions and measured torque is approximated linearly.

A torsion spring 1 to be tested is inserted between a driving plate 52 and a plate 56 at a test fixture 20 so as to be positioned on a spindle. Then, the driving plate 52 is caused to rotate by a motor 30. Electric pulses are generated by an encoder 32 at every 7 degrees/20 pulse. The spring 1 is driven by a portion of 280 pulses (98 degrees) in its winding direction. After that, it is driven by the 280 pulse portion in its unwinding direction. A torque which is detected by a load cell 66 at every signal from the encoder 32 is recorded. Then, a working curve which expresses the relationship between a plurality of established winding and unwinding positions (pulse data) and the torque is generated, and the spring 1 is graded. The torque of the spring 1 which is suitable for intended use is set within 0.048±0.003 ounce-inch.