A construction machine comprising: a lower traveler; an upper slewing body mounted on the lower traveler so as to be capable of being slewed; an engine having an output shaft; an engine assist motor connected to the output shaft of the engine to be capable of assisting the engine with a consumption of an electric power; a slewing electric motor for slewing the upper slewing body and regenerating a regeneration power when the upper slewing body is braked; a regeneration power detector for detecting the regeneration power; and an electrical storage device electrically connected to the engine assist motor and the slewing electric motor to be capable of being charged with the regeneration power and discharging electricity; characterized in that the construction machine further comprises: a charging rate detector for detecting a charging rate of the electrical storage device; a maximum-allowable-charge-amount calculator for calculating a maximum allowable charge amount which is a maximum amount of the electric power within which the electrical storage device is allowed to be charged, on the basis of the charging rate detected by the charging rate detector; and an assist controller which causes the engine assist motor to assist the engine to consume an electric power equal to or more than an electric power equal to a difference between the regeneration power and the maximum allowable charge amount, in a case where the regeneration power detected by the regeneration power detector is equal to or more than the maximum allowable charge amount calculated by the maximum-allowable-charge-amount calculator. a lower traveler; an upper slewing body mounted on the lower traveler so as to be capable of being slewed; an engine having an output shaft; an engine assist motor connected to the output shaft of the engine to be capable of assisting the engine with a consumption of an electric power; a slewing electric motor for slewing the upper slewing body and regenerating a regeneration power when the upper slewing body is braked; a regeneration power detector for detecting the regeneration power; and an electrical storage device electrically connected to the engine assist motor and the slewing electric motor to be capable of being charged with the regeneration power and discharging electricity; characterized in that the construction machine further comprises: a charging rate detector for detecting a charging rate of the electrical storage device; a maximum-allowable-charge-amount calculator for calculating a maximum allowable charge amount which is a maximum amount of the electric power within which the electrical storage device is allowed to be charged, on the basis of the charging rate detected by the charging rate detector; and an assist controller which causes the engine assist motor to assist the engine to consume an electric power equal to or more than an electric power equal to a difference between the regeneration power and the maximum allowable charge amount, in a case where the regeneration power detected by the regeneration power detector is equal to or more than the maximum allowable charge amount calculated by the maximum-allowable-charge-amount calculator. a charging rate detector for detecting a charging rate of the electrical storage device; a maximum-allowable-charge-amount calculator for calculating a maximum allowable charge amount which is a maximum amount of the electric power within which the electrical storage device is allowed to be charged, on the basis of the charging rate detected by the charging rate detector; and an assist controller which causes the engine assist motor to assist the engine to consume an electric power equal to or more than an electric power equal to a difference between the regeneration power and the maximum allowable charge amount, in a case where the regeneration power detected by the regeneration power detector is equal to or more than the maximum allowable charge amount calculated by the maximum-allowable-charge-amount calculator.