A construction machine having a platform for maintenance work, comprising: a lower propelling body; an upper slewing body mounted on the lower propelling body through a slewing bearing slewably about an axis perpendicular to a ground surface, the upper slewing body having a rear region provided with an engine room having an upward-facing opening and housing a plurality of equipment including an engine, and an engine cover for selectively opening and closing the opening of the engine room; and a slewing device for slewing the upper slewing body, the slewing device including: a slewing electric motor having an electric-motor output shaft; and a platform cover, characterized in that the slewing device further includes a speed reducer having a speed-reducer output shaft, the speed reducer being coupled to the electric-motor output shaft such that the speed-reducer output shaft is rotated, according to rotation of the electric-motor output shaft, at a speed lower than that of the rotation; wherein: the speed reducer is installed forward of the engine room in such a posture that the speed-reducer output shaft points downward; the slewing electric motor is disposed just above the speed reducer; and the platform cover is provided on a top of the slewing electric motor, and has a flat upper surface serving as a platform for maintenance work of equipment housed in the engine room, and wherein the slewing device is disposed such that the upper surface of the platform cover is located at a position lower than the engine cover and adjacent to a front side of the engine room. a lower propelling body; an upper slewing body mounted on the lower propelling body through a slewing bearing slewably about an axis perpendicular to a ground surface, the upper slewing body having a rear region provided with an engine room having an upward-facing opening and housing a plurality of equipment including an engine, and an engine cover for selectively opening and closing the opening of the engine room; and a slewing device for slewing the upper slewing body, the slewing device including: a slewing electric motor having an electric-motor output shaft; and a platform cover, characterized in that the slewing device further includes a speed reducer having a speed-reducer output shaft, the speed reducer being coupled to the electric-motor output shaft such that the speed-reducer output shaft is rotated, according to rotation of the electric-motor output shaft, at a speed lower than that of the rotation; wherein: the speed reducer is installed forward of the engine room in such a posture that the speed-reducer output shaft points downward; the slewing electric motor is disposed just above the speed reducer; and the platform cover is provided on a top of the slewing electric motor, and has a flat upper surface serving as a platform for maintenance work of equipment housed in the engine room, and wherein the slewing device is disposed such that the upper surface of the platform cover is located at a position lower than the engine cover and adjacent to a front side of the engine room. the speed reducer is installed forward of the engine room in such a posture that the speed-reducer output shaft points downward; the slewing electric motor is disposed just above the speed reducer; and the platform cover is provided on a top of the slewing electric motor, and has a flat upper surface serving as a platform for maintenance work of equipment housed in the engine room, and wherein the slewing device is disposed such that the upper surface of the platform cover is located at a position lower than the engine cover and adjacent to a front side of the engine room.