A construction machine comprising: a frame; an engine installed on the frame; a hydraulic pump installed on the frame in side-by-side relation with the engine along an axial direction of the engine to be driven by the engine; a guard panel disposed on the frame so as to surround an engine room in which the engine and the hydraulic pump is accommodated, the guard panel having a maintenance port which is an opening for allowing maintenance of the hydraulic pump to be performed from the outside; and an exhaust-gas-purification device provided in the engine room and connected to the engine via an exhaust pipe to remove particulate matter contained in exhaust gas from the engine; characterized by a support member adapted to support an exhaust-gas-aftertreatment device including the exhaust-gas-purification device, inside the engine room at a position just above the hydraulic pump, the support member including: a purification device supporting portion to support the exhaust-gas-purification device; and a catalyst supporting portion to support, at a position closer to the maintenance port than the exhaust-gas-purification device, a NOx-reducing catalyst which is equipment to be additionally included in the gas aftertreatment device so as to reduce NOx contained in the exhaust gas, the catalyst supporting portion being adapted to support the NOx-reducing catalyst at a position higher than the exhaust-gas-purification device supported by the purification device supporting portion, so as to form, just above the hydraulic pump, a maintenance space having an upper end at a position higher than an upper end of a space formed just below the purification device supporting portion and the exhaust-gas-purification device supported by the purification device supporting portion. a frame; an engine installed on the frame; a hydraulic pump installed on the frame in side-by-side relation with the engine along an axial direction of the engine to be driven by the engine; a guard panel disposed on the frame so as to surround an engine room in which the engine and the hydraulic pump is accommodated, the guard panel having a maintenance port which is an opening for allowing maintenance of the hydraulic pump to be performed from the outside; and an exhaust-gas-purification device provided in the engine room and connected to the engine via an exhaust pipe to remove particulate matter contained in exhaust gas from the engine; characterized by a support member adapted to support an exhaust-gas-aftertreatment device including the exhaust-gas-purification device, inside the engine room at a position just above the hydraulic pump, the support member including: a purification device supporting portion to support the exhaust-gas-purification device; and a catalyst supporting portion to support, at a position closer to the maintenance port than the exhaust-gas-purification device, a NOx-reducing catalyst which is equipment to be additionally included in the gas aftertreatment device so as to reduce NOx contained in the exhaust gas, the catalyst supporting portion being adapted to support the NOx-reducing catalyst at a position higher than the exhaust-gas-purification device supported by the purification device supporting portion, so as to form, just above the hydraulic pump, a maintenance space having an upper end at a position higher than an upper end of a space formed just below the purification device supporting portion and the exhaust-gas-purification device supported by the purification device supporting portion.