PURPOSE: To improve mechanism strength of a shaft, and eliminate mechanical fatigue from the small diameter part of the shaft by specifying the cross section of the fluid passage of a shaft core communicated with an upper part penetrating hole, and thickening the wall thickness of the small diameter part upper part of the shaft on which the upper part penetrating hole is drilled.

CONSTITUTION: The fluid passage 20 of the core of a shaft 2 is a small diameter passage 21 whose inner diameter is reduced to half calculated in cross-section equivalent in the upper side of a shaft small diameter part 19. Two lower part penetrating holes 22, 22 are drilled and communicated with the passage 20 in left and right directions on the lower side of the small diameter part 19, and two upper part penetrating holes 23, 23 are drilled and communicated with a small diameter flow passage 21 in a direction perpendicular to the drilling direction of the penetrating holes 22, 22. The half of high pressure fluid which flow a fluid passage 20 flow out from the lower part penetrating holes 22, 22, and thereby, the cross section of the fluid passage which is communicated with the upper part penetrating holes 23, 23 is reduced to the half. It is thus possible to form the fluid passage in which the upper penetrating holes 23, 23 are formed as the small diameter flow passage 21, and also it is possible to thicken the wall of an upper side pipe wall 24.