PURPOSE: To keep off any damage due to corrosion fatigue in a weld zone, by screwing a choke plug in the top of a hollow pump shaft and welding it, while installing grooves in position approaching closer to the weld zone, at both inner and outer sides of the weld zone.

CONSTITUTION: A choke plug 5 is screwed in the top of a hollow part of a pump shaft 4 and solidly coupled with the pump shaft 4 through a weld zone 10. In one side or both at the inner and outer sides of the weld zone 10 between the pump shaft 4 and the choke plug 5, there are provided with circular grooves 11a and 11b in close vicinity to these ones. Therefore, welding heat in time of weld is intercepted by these grooves 11a and 11b so that a heat effect is limited to space between these grooves 11a and 11b, thus no effect is given to other parts.