A hybrid construction machine having a hydraulic actuator, comprising a battery; an engine as a power source; a hydraulic pump which is connected to the engine as a hydraulic source for the hydraulic actuator; a generator motor which is connected to the engine, charges the battery by acting as a generator by power from the engine and assists the engine by acting as an electric motor by electric power from the battery; a voltage detector which detects terminal voltage of the battery; and a current detector which detects charge current and discharge current of the battery, characterized by a controller (14, 15) which is configured to calculate a present charging rate as a currently remaining charged amount of the battery, wherein the controller (14, 15) is configured to switch over a calculation mode of calculating the present charging rate between a current integration mode, in which the charge current and the discharge current of the battery detected by the current detector are integrated and the integrated value of the charge current is added to an initial charging rate of the battery as well as the integrated value of the discharge current is subtracted from the initial charging rate of the battery to calculate the present charging rate, and a voltage reference mode in which the present charging rate is determined based on a predefined relation between terminal voltage and charging rate of the battery and terminal voltage of the battery detected by the voltage detector; calculate, during operation of the generator motor, the present charging rate according to the current integration mode, and suspend operation of the generator motor at a predetermined switchover timing and calculate the present charging rate according to the voltage reference mode; and after calculation of the present charging rate according to the voltage reference mode, restore the calculation mode back to the current integration mode from the voltage reference mode and cancel the suspension of operating the generator motor, and wherein the controller (14, 15) is configured to measure an accumulated operation time of the hybrid construction machine as well as to define the switchover timing based on a timing when the accumulated operation time reaches a predetermined value. switch over a calculation mode of calculating the present charging rate between a current integration mode, in which the charge current and the discharge current of the battery detected by the current detector are integrated and the integrated value of the charge current is added to an initial charging rate of the battery as well as the integrated value of the discharge current is subtracted from the initial charging rate of the battery to calculate the present charging rate, and a voltage reference mode in which the present charging rate is determined based on a predefined relation between terminal voltage and charging rate of the battery and terminal voltage of the battery detected by the voltage detector; calculate, during operation of the generator motor, the present charging rate according to the current integration mode, and suspend operation of the generator motor at a predetermined switchover timing and calculate the present charging rate according to the voltage reference mode; and after calculation of the present charging rate according to the voltage reference mode, restore the calculation mode back to the current integration mode from the voltage reference mode and cancel the suspension of operating the generator motor, and