A swiveling work machine includes a swivel deck, an engine and an operator's seat which are mounted on the swivel deck and a protector for the operator's seat mounted erect on the swivel deck. The swivel deck mounts, at a rear portion thereof, a support frame having four post portions disposed around the engine. Two rear post portions of the support frame have upper ends thereof connected via a rear beam member. The rear beam member includes an attaching portion for the operator's seat protector to which portion the protector is attached.