The present invention includes a target rotation speed setup section (17) for setting a target rotation speed of an engine (1); load detection means (21) for detecting a load on a hydraulic pump (3); an assist output computation section (19) for calculating an assist output to be generated by a motor generator in accordance with a rotation speed deviation ΔN, which is the difference between an actual rotation speed and the target rotation speed, or in accordance with the load on the hydraulic pump; an absorption torque upper limit computation section (23) for calculating an absorption torque upper limit value of the hydraulic pump (3); and an operation signal generation section (24) for generating the operation signal to be output to a pump displacement adjustment device (45). When the rotation speed deviation ΔN is equal to or more that a setting NC which is determined in accordance with the assist output value, the absorption torque upper limit computation section reduces the absorption torque upper limit value of the hydraulic pump from the calculated value.