A construction machine having an engine, an assist electric motor that is coaxially and mechanically connected to the engine, and an electrical power converter (9, 13) that is connected to the assist electric motor and to an electrical storage device in order to control the voltage of the assist electric motor and the voltage of the electrical storage device, the construction machine comprising: charge/discharge control means for controlling a charge/discharge amount in accordance with a charge amount target value (Q*), the charge amount target value (Q*) being set to be equal to a minimum charge amount (Qmin) of the electrical storage device at the end of work, characterized in that the construction machine further comprises: a charging device connected to the electrical storage device, the electrical storage device being capable of becoming charged through the charging device from an external power source; and wherein the charge/discharge control means includes a charge amount target value computation section that computes the charge amount target value (Q*) for the electrical storage device in accordance with work time of the construction machine and with the predefined estimated work time (Ts), and a charge/discharge target value computation section (305, 306) that discharges the electrical storage device when a charge amount of the electrical storage device is larger than a charge amount (Q* + Qa) determined in accordance with the charge amount target value (Q*) and with a predetermined value (Qa), and charges the electrical storage device when the charge amount of the electrical storage device is smaller than a charge amount (Q* - Qc) determined in accordance with the charge amount target value (Q*) and with a predetermined value (Qc). charge/discharge control means for controlling a charge/discharge amount in accordance with a charge amount target value (Q*), the charge amount target value (Q*) being set to be equal to a minimum charge amount (Qmin) of the electrical storage device at the end of work, characterized in that the construction machine further comprises: a charging device connected to the electrical storage device, the electrical storage device being capable of becoming charged through the charging device from an external power source; and wherein the charge/discharge control means includes a charge amount target value computation section that computes the charge amount target value (Q*) for the electrical storage device in accordance with work time of the construction machine and with the predefined estimated work time (Ts), and a charge/discharge target value computation section (305, 306) that discharges the electrical storage device when a charge amount of the electrical storage device is larger than a charge amount (Q* + Qa) determined in accordance with the charge amount target value (Q*) and with a predetermined value (Qa), and charges the electrical storage device when the charge amount of the electrical storage device is smaller than a charge amount (Q* - Qc) determined in accordance with the charge amount target value (Q*) and with a predetermined value (Qc). a charging device connected to the electrical storage device, the electrical storage device being capable of becoming charged through the charging device from an external power source; and wherein the charge/discharge control means includes a charge amount target value computation section that computes the charge amount target value (Q*) for the electrical storage device in accordance with work time of the construction machine and with the predefined estimated work time (Ts), and a charge/discharge target value computation section (305, 306) that discharges the electrical storage device when a charge amount of the electrical storage device is larger than a charge amount (Q* + Qa) determined in accordance with the charge amount target value (Q*) and with a predetermined value (Qa), and charges the electrical storage device when the charge amount of the electrical storage device is smaller than a charge amount (Q* - Qc) determined in accordance with the charge amount target value (Q*) and with a predetermined value (Qc).