PURPOSE: To surely and smoothly slide a ball bearing pre-load being applied thereto onto a positive electrode fixing base by providing an auxiliary spring applying a pressure in the reverse direction to a pre-load direction of a pre-load spring at the ball bearing pre-load being applied thereto at its opposite side of the auxiliary spring.

CONSTITUTION: A coil auxiliary spring 27 is fitted to the periphery of a rotary shaft 18 in an outer ring lace 26 of a ball bearing 23 located far from a positive electrode target. The upper end portion of the coil auxiliary spring 27 is attached to an inner step portion of a cylindrical portion 19a of a positive electrode fixing member 19. On the other hand, a lower end portion thereof is attached to an upper face of the outer ring lace 26 of the ball bearing 23. A space is provided between the ball bearing 23 and a bottom portion 19b of the positive electrode fixing member 19. A coil auxiliary spring 42 is further disposed in the space. Thus, pre-load in a thrust direction is divided and applied to a pair of ball bearings 22 and 23. Therefore, the ball bearings 22 and 23 the pre-load being applied thereto are always surely and smoothly slided onto the positive electrode fixing member 19 without scratch and bite.