A hybrid construction machine comprising: a hydraulic pump that serves as a hydraulic power source of a hydraulic actuator; a generator motor; an engine that is connected to the hydraulic pump and the generator motor; a power storage device; a generator motor controller that assists the engine by a motor action of the generator motor with the aid of power of the power storage device; a control unit that controls charging and discharging of the power storage device; and a battery monitoring device that detects a voltage of the power storage device, wherein characterized in that the control unit includes: a difference voltage calculating unit that calculates a difference voltage ( ΔV), which is a difference between an upper-limit working voltage that is set in advance as an upper limit of a usable voltage of the power storage device, and the voltage of the power storage device detected by the battery monitoring device; a storage unit that stores the difference voltage ( ΔV); an updating unit that updates the difference voltage ( ΔV) which stored by the storage unit by selecting the lower one of the calculated difference voltage and the stored difference voltage stored in the storage unit; a correcting unit that corrects an upper-limit working state of charge SOC, which is set in advance as an upper limit of an SOC usable for the power storage device, to decrease the upper-limit working SOC as the selected difference voltage decreases; and a charging/discharging control unit that controls charging of the power storage device, based on the corrected upper-limit working SOC. a hydraulic pump that serves as a hydraulic power source of a hydraulic actuator; a generator motor; an engine that is connected to the hydraulic pump and the generator motor; a power storage device; a generator motor controller that assists the engine by a motor action of the generator motor with the aid of power of the power storage device; a control unit that controls charging and discharging of the power storage device; and a battery monitoring device that detects a voltage of the power storage device, wherein characterized in that the control unit includes: a difference voltage calculating unit that calculates a difference voltage ( ΔV), which is a difference between an upper-limit working voltage that is set in advance as an upper limit of a usable voltage of the power storage device, and the voltage of the power storage device detected by the battery monitoring device; a storage unit that stores the difference voltage ( ΔV); an updating unit that updates the difference voltage ( ΔV) which stored by the storage unit by selecting the lower one of the calculated difference voltage and the stored difference voltage stored in the storage unit; a correcting unit that corrects an upper-limit working state of charge SOC, which is set in advance as an upper limit of an SOC usable for the power storage device, to decrease the upper-limit working SOC as the selected difference voltage decreases; and a charging/discharging control unit that controls charging of the power storage device, based on the corrected upper-limit working SOC. a difference voltage calculating unit that calculates a difference voltage ( ΔV), which is a difference between an upper-limit working voltage that is set in advance as an upper limit of a usable voltage of the power storage device, and the voltage of the power storage device detected by the battery monitoring device; a storage unit that stores the difference voltage ( ΔV); an updating unit that updates the difference voltage ( ΔV) which stored by the storage unit by selecting the lower one of the calculated difference voltage and the stored difference voltage stored in the storage unit; a correcting unit that corrects an upper-limit working state of charge SOC, which is set in advance as an upper limit of an SOC usable for the power storage device, to decrease the upper-limit working SOC as the selected difference voltage decreases; and a charging/discharging control unit that controls charging of the power storage device, based on the corrected upper-limit working SOC.