A control device for a construction machine having an engine, a variable displacement hydraulic pump driven by the engine, a hydraulic actuator driven by a hydraulic fluid discharged from the hydraulic pump, a motor generator for exchanging torque with the engine, electrical storage means for supplying electric power to the motor generator, and pump displacement adjustment means for adjusting the displacement of the hydraulic pump in accordance with an operation signal, the control device comprising: actual rotation speed detection means that detects an actual rotation speed of the engine; target rotation speed setup means that sets a target rotation speed of the engine; assist output computation means that calculates an assist output value that is to be generated by the motor generator in order to assist the output of the engine; absorption torque upper limit computation means that calculates an absorption torque upper limit value of the hydraulic pump; and operation signal generation means that generates the operation signal to be output to the pump displacement adjustment means for the purpose of adjusting the displacement of the hydraulic pump in accordance with the value calculated by the absorption torque upper limit computation means; characterized in that, when a rotation speed deviation indicative of the difference between the actual rotation speed, the data about the actual rotation speed being input from the actual rotation speed detection means, and the target rotation speed, the data about the target rotation speed being input from the target rotation speed setup means, is equal to or more that a setting value that is determined in accordance with the assist output value calculated by the assist output computation means, the absorption torque upper limit computation means reduces the absorption torque upper limit value of the hydraulic pump from the calculated value. actual rotation speed detection means that detects an actual rotation speed of the engine; target rotation speed setup means that sets a target rotation speed of the engine; assist output computation means that calculates an assist output value that is to be generated by the motor generator in order to assist the output of the engine; absorption torque upper limit computation means that calculates an absorption torque upper limit value of the hydraulic pump; and operation signal generation means that generates the operation signal to be output to the pump displacement adjustment means for the purpose of adjusting the displacement of the hydraulic pump in accordance with the value calculated by the absorption torque upper limit computation means; characterized in that, when a rotation speed deviation indicative of the difference between the actual rotation speed, the data about the actual rotation speed being input from the actual rotation speed detection means, and the target rotation speed, the data about the target rotation speed being input from the target rotation speed setup means, is equal to or more that a setting value that is determined in accordance with the assist output value calculated by the assist output computation means, the absorption torque upper limit computation means reduces the absorption torque upper limit value of the hydraulic pump from the calculated value.