A swivel work machine includes a traveling unit, a swivel base plate supported on the traveling unit to be pivotable about a vertical swivel axis, the swivel base plate having a first lateral side and a second lateral side provided across said vertical axis, a pair of upper and lower support brackets disposed at a front end of the swivel base plate with an offset toward the first lateral side and adapted for supporting an implement, a pair of left and right vertical ribs extending rearward from the support brackets and fixed to the swivel base plate and a cabin mounted on the swivel base plate. The cabin is disposed with an offset toward the second lateral side relative to the support brackets, a bottom of the cabin being disposed downwardly of the upper support bracket. The vertical rib disposed on the side of the second lateral side extends, from its front portion to its intermediate portion, parallel with a side face of the cabin on the side of the first lateral side.