The degradation of an electrical storage device 30 such as a capacitor can be determined by a system which is not an external device but is installed in hybrid construction equipment 1 in a state that the electrical storage device 30 such as a capacitor is mounted. To achieve this objective, it is at least checked that the working machine has stopped. Next, when it has been checked that the working machine has stopped, a degradation state determination time control mode is entered, a motor-generator is driven at the prescribed rotational speed and the prescribed torque, and the electrical storage device rises from the prescribed start charging voltage to the prescribed end charging voltage (Step 102). Next, the capacitance of the electrical storage device is calculated based on the values of the rotational speed and the torque of the motor-generator during the degradation state determination time control mode, the values of the start charging voltage and the end charging voltage of the electrical storage device, and the time needed for the electrical storage device to reach the end charging voltage form the start charging voltage. Next, by comparing the calculated capacitance of the electrical storage device and the reference capacitance, the degradation state of the electrical storage device is determined.