A control device for a construction machine provided with a machine body, a boom configured to be raised and lowered with respect to the machine body, and an arm configured to be swingable with respect to the boom, the control device comprising: a boom cylinder which raises and lowers the boom; an arm cylinder which swings the arm; a variable capacity hydraulic pump which supplies hydraulic oil to the arm cylinder; an engine which drives the hydraulic pump; a rotation number designating unit which outputs a command for designating the number of rotations of the engine; a regeneration valve which is switchable between a regeneration state, in which return oil from the boom cylinder at a time of lowering the boom is drawn into a supply side port of the arm cylinder at a time of pressing the arm, and a closed state, in which drawing of the return oil into the arm cylinder is prevented; flow rate detecting means which is configured to detect a value for specifying an ejection flow rate of the hydraulic pump; and a controller which controls an operation of the regeneration valve so that the regeneration valve is switched to the regeneration state, and controls a flow rate of the hydraulic pump so as to reduce the ejection flow rate of the hydraulic pump in accordance with regeneration of hydraulic oil through the regeneration valve at a time of a combined operation of lowering the boom and pressing the arm, characterized in that the controller outputs a command for setting the number of rotations of the engine to be smaller than the rotation number designated by the rotation number designating unit when the ejection flow rate of the hydraulic pump detected by the flow rate detecting means is not larger than a predetermined flow rate at the time of the combined operation, and the predetermined flow rate is a flow rate when the tilt of the hydraulic pump is minimized in a state that the engine is driven at the rotation number designated by the rotation number designation unit. a boom cylinder which raises and lowers the boom; an arm cylinder which swings the arm; a variable capacity hydraulic pump which supplies hydraulic oil to the arm cylinder; an engine which drives the hydraulic pump; a rotation number designating unit which outputs a command for designating the number of rotations of the engine; a regeneration valve which is switchable between a regeneration state, in which return oil from the boom cylinder at a time of lowering the boom is drawn into a supply side port of the arm cylinder at a time of pressing the arm, and a closed state, in which drawing of the return oil into the arm cylinder is prevented; flow rate detecting means which is configured to detect a value for specifying an ejection flow rate of the hydraulic pump; and a controller which controls an operation of the regeneration valve so that the regeneration valve is switched to the regeneration state, and controls a flow rate of the hydraulic pump so as to reduce the ejection flow rate of the hydraulic pump in accordance with regeneration of hydraulic oil through the regeneration valve at a time of a combined operation of lowering the boom and pressing the arm, characterized in that the controller outputs a command for setting the number of rotations of the engine to be smaller than the rotation number designated by the rotation number designating unit when the ejection flow rate of the hydraulic pump detected by the flow rate detecting means is not larger than a predetermined flow rate at the time of the combined operation, and the predetermined flow rate is a flow rate when the tilt of the hydraulic pump is minimized in a state that the engine is driven at the rotation number designated by the rotation number designation unit.