A swiveling work machine includes a swivel deck mounted to be pivotable about a vertical axis and a side cover for covering one lateral side on the swivel deck, an accommodating space capable of accommodating a work machine accessory being provided inside the lateral cover. The accommodating space accommodates, as the implement accessory, a work oil tank, a fuel tank and a control valve unit. The work oil tank is disposed at a fore-and-aft intermediate portion on one lateral side on the swivel deck. The fuel tank is disposed forwardly of the work oil tank with forming a gap relative thereto. The control valve unit is disposed upwardly of the fuel tank and longitudinally along the fore/aft direction.