To improve the design freedom and attaching strength by strongly attaching a revolving bearing to a bottom plate at a free position regardless of the position of a vertical plate of a revolving frame.

On a right vertical plate 13 of a revolving frame 10, a notched part 15 opened to the lower side at a position facing each female screw hole 11C provided on a bottom plate 11 is provided. Thus, even when providing the female screw hole 11C on the lower side of the right vertical plate 13, since the notched part 15 can evade the distal end part of a bolt 18 projected to the upper side from the female screw hole 11C, the female screw hole 11c is freely arranged at a position at which high attaching strength can be obtained. Also, a reinforcing block 17 is provided so as to fill a space part 16 formed between the notched part 15 and an upper surface 11A of the bottom plate 11, and the reinforcing block 17 is fixed to the bottom plate 11 and the right vertical plate 13 by using welding means. Thus, concentration of large loads on the notched part 15 is prevented.