Abstract: An upper cover has a curved portion formed in its end portion on the side near a work front for avoiding interference with the work front. The curved portion comprises a recessed surface having a center axis aligned with the axis of a vertical pin of a swing post, a first stepped portion provided at a lower end of the recessed surface and defining a substantially constant gap with respect to a locus drawn by a rear projecting portion of the swing post when the work front is swung, and a second stepped portion provided in an intermediate portion of the recessed surface in the direction of height thereof and having a substantially horizontal surface for assuring a foothold. The recessed surface is configured to extend along a locus drawn by a back surface of the boom when the work front is swung in its minimum-turn posture, while defining a gap with respect to the locus.