To improve production efficiency by dispensing with labor, lowering production cost and shortening manufacturing time.

Grasping units 392a, 392b, 394a, 394b grasping a spring 229 and retainers 78a, 78b are displaced to the arrow C direction and valve stems 444a, 444b are inserted into the inner side of the spring 229. The grasping units 392a, 392b, 394a, 394b are then displaced a little to the arrow I direction and a little further to the arrow C direction and the valve stems 444a, 444b are inserted through large diameters 462a, 462b of holes 458a, 458b which are divided into retainers 78a, 78b. The grasping units 392a, 392b, 394a, 394b are then displaced a little to the arrow H direction and a groove 446 of valve stems 444a, 444b is engaged with small diameters 460a, 460b continuously formed on the large diameters 462a, 462b.