To surely perform positioning of the bolt holes of a rotating frame side and the bolt holes of a weight side in the case that a counter weight is attached to the rotating frame.

Three guide projections 61, 62 and 63 are provided so that the circumference of a weight housing recess part 25B is surrounded on an upper plate 25 of a rear frame 24, and when a hung center low weight 42 is mounted on the upper plate 25 of the rear frame 24, weight side bolt holes 43A are automatically aligned in frame side bolt holes 25C of the upper plate 25 by guiding the end edge part of a flange plate 43 on inclined faces 61B-63B of the guide projections 61-63. Thereby while the hung center low weight 42 is mounted on the upper plate 25 of the rear frame 24, the weight side bolt holes 43A can be automatically aligned in the frame side bolt holes 25C, and workability during the time when the center low weight 42 is attached to the rear frame 24 by using bolts 45 can be enhanced.