When temperature is low, a low-temperature mode is selected, followed by the activation of the heating mode functionality 25a of a PCU 25. Specifically, the PCU 25 sets the ratio of energy recovered by a capacitor 23 to be equal to or greater than the largest value of the energy recovery ratios of the capacitor 23 in a normal mode (e.g., assist motor 22:capacitor 23=2:8). Thus, more energy is stored on the capacitor 23 than in the normal mode, helping the capacitor 23 generates heat. As a result, the liquid temperature of the coolant is increased, and its viscosity is reduced accordingly. As above, the invention provides a simple cooling system to be mounted on a hybrid working machine, which is capable of preventing decreases in energy efficiency at low temperature.