PURPOSE: To guide a housing yoke smoothly in the vertical direction in spite of slight deformation of the housing yoke by checking only the movement in the longitudinal direction of the housing yoke by a guide block to move vertically along an auxiliary guide post.

CONSTITUTION: The first linear guide rail 35 is arranged in the vertical direction along an auxiliary guide post 7, and the second linear guide rail 47 is also arranged in the first guide block 37 to move vertically along this first linear guide rail 35. This second linear guide rail 47 is arranged in the direction orthogonal to the first linear guide rail 35, and the second guide block 49 can move freely in the lateral direction. This second guide block 49 is installed in a housing yoke 9. Thereby, even if lateral directional force acts by deformation of the housing yoke 9, since the second guide block 49 moves, the lateral directional force does not act on the first guide block 37.