Provided is a construction machine which comprises an upper slewing body including: a frame bottom plate formed with an downward opening; an undercover detachably attached to the frame bottom plate to close up the opening; and a connection member for connecting the undercover to the frame bottom plate. The undercover includes a cover-forming member made of a single plate fabricated to have a base portion extending along a lower surface of the frame bottom plate and a bulging portion bulging upwardly from the base portion to be inserted into the opening. The base portion has a contact area in an outer edge region thereof to make contact with the lower surface of the frame bottom plate from the lower side thereof. The connection member connects the bulging portion to a region of the frame bottom plate, the region located above the lower surface of the frame bottom plate.