An assembly method includes a step of press-fitting a second valve 27 to a valve shaft 29 to which a first valve 25 is press-fitted and secured while measuring and adjusting a distance L2 so that it becomes equal to a distance L1. Thus, the first valve 25 and second valve 27 are press-fitted to the valve shaft 29 and those components are swaged together so that the first and second valves are secured to the valve shaft, and oil is placed in a press-fitted portion. When components, such as the first valve 25, are sintered parts, oil contained in the sintered parts themselves is used for increasing a joining force that occurs in the press-fitted portion.