A hybrid construction machine comprising: an engine; an electric motor/generator that assists power of the engine during power running and generates electricity during regenerative braking; a hydraulic pump unit that is driven by the power of the engine and delivers a hydraulic fluid; an electricity storage device; a power converter that converts direct-current power that has been received from the electricity storage device during the power running into alternating-current power and supply the alternating-current power to the electric motor/generator, the power converter converting the alternating-current power that the electric motor/generator has generated into direct-current power and supplying the direct-current power to the electricity storage device; and a temperature regulator that regulates a temperature of the electricity storage device; characterized in that the hybrid construction machine further comprises a control unit that calculates a charge/discharge time ratio denoting a rate of, with respect to the certain period of time, a sum of a period of time during which the current value of the charge/discharge current is equal to or greater than a first threshold level Ld, and a period of time during which the current value of the charge/discharge current is equal to or smaller than a second threshold level Lc that is smaller than the first threshold level from the charge/discharge history that is data on charge/discharge current of the electricity storage device, the data being measured during a certain period of time in a past and executes at least one of first control, second control, and third control in accordance with the charge/discharge time ratio, the first control controlling the temperature regulator so that the temperature of the electricity storage device increases, the second control controlling the power converter so that the power that is output from the power converter is reduced, the third control controlling the hydraulic pump unit so that a flow rate of the fluid delivered from the hydraulic pump unit is reduced. an engine; an electric motor/generator that assists power of the engine during power running and generates electricity during regenerative braking; a hydraulic pump unit that is driven by the power of the engine and delivers a hydraulic fluid; an electricity storage device; a power converter that converts direct-current power that has been received from the electricity storage device during the power running into alternating-current power and supply the alternating-current power to the electric motor/generator, the power converter converting the alternating-current power that the electric motor/generator has generated into direct-current power and supplying the direct-current power to the electricity storage device; and a temperature regulator that regulates a temperature of the electricity storage device; characterized in that the hybrid construction machine further comprises a control unit that calculates a charge/discharge time ratio denoting a rate of, with respect to the certain period of time, a sum of a period of time during which the current value of the charge/discharge current is equal to or greater than a first threshold level Ld, and a period of time during which the current value of the charge/discharge current is equal to or smaller than a second threshold level Lc that is smaller than the first threshold level from the charge/discharge history that is data on charge/discharge current of the electricity storage device, the data being measured during a certain period of time in a past and executes at least one of first control, second control, and third control in accordance with the charge/discharge time ratio, the first control controlling the temperature regulator so that the temperature of the electricity storage device increases, the second control controlling the power converter so that the power that is output from the power converter is reduced, the third control controlling the hydraulic pump unit so that a flow rate of the fluid delivered from the hydraulic pump unit is reduced.