Hybrid construction machine comprising: an engine including a crankshaft; a hydraulic pump connected to the engine and configured to drive a hydraulic actuator; a generator motor connected to the engine and configured to operate as a generator by power of the engine to thereby charge an electric storage device and operate as a motor by electric power of the electric storage device to thereby assist the engine; a flywheel attached to the crankshaft; and a coupling connecting a pump shaft included in the hydraulic pump to a rotor shaft included in the generator motor, wherein the generator motor includes: a rotor configured to rotate about the rotor shaft; and a stator having a coil arranged in an outer circumference of the rotor, and (i) the crankshaft and the flywheel are directly coupled to each other, and the flywheel and the rotor shaft are directly coupled to each other, (ii) the coupling is directly coupled to an end face of the rotor shaft on an opposite side to the flywheel, and the pump shaft is connected to the coupling, (iii) coupling surfaces of the flywheel and the rotor shaft, and coupling surfaces of the rotor shaft and the coupling are arranged within a width range of the coil in the axial direction of the rotor shaft, wherein a circular recess centering on a rotation center of the rotor shaft is provided on the coupling surface of the flywheel with respect to the rotor shaft, a ring-like protrusion centering on the rotation center of the rotor shaft is provided on the coupling surface of the rotor shaft with respect to the flywheel, the hybrid construction machine further comprises flywheel attachment bolts that penetrates a bottom wall of the recess and are screwed into an end face of the crankshaft to thereby couple the flywheel to the crankshaft, and the flywheel and the rotor shaft are coupled to each other in a state that the protrusion is fit in the recess. an engine including a crankshaft; a hydraulic pump connected to the engine and configured to drive a hydraulic actuator; a generator motor connected to the engine and configured to operate as a generator by power of the engine to thereby charge an electric storage device and operate as a motor by electric power of the electric storage device to thereby assist the engine; a flywheel attached to the crankshaft; and a coupling connecting a pump shaft included in the hydraulic pump to a rotor shaft included in the generator motor, wherein the generator motor includes: a rotor configured to rotate about the rotor shaft; and a stator having a coil arranged in an outer circumference of the rotor, and (i) the crankshaft and the flywheel are directly coupled to each other, and the flywheel and the rotor shaft are directly coupled to each other, (ii) the coupling is directly coupled to an end face of the rotor shaft on an opposite side to the flywheel, and the pump shaft is connected to the coupling, (iii) coupling surfaces of the flywheel and the rotor shaft, and coupling surfaces of the rotor shaft and the coupling are arranged within a width range of the coil in the axial direction of the rotor shaft, wherein (i) the crankshaft and the flywheel are directly coupled to each other, and the flywheel and the rotor shaft are directly coupled to each other, (ii) the coupling is directly coupled to an end face of the rotor shaft on an opposite side to the flywheel, and the pump shaft is connected to the coupling, (iii) coupling surfaces of the flywheel and the rotor shaft, and coupling surfaces of the rotor shaft and the coupling are arranged within a width range of the coil in the axial direction of the rotor shaft, wherein a circular recess centering on a rotation center of the rotor shaft is provided on the coupling surface of the flywheel with respect to the rotor shaft, a ring-like protrusion centering on the rotation center of the rotor shaft is provided on the coupling surface of the rotor shaft with respect to the flywheel, the hybrid construction machine further comprises flywheel attachment bolts that penetrates a bottom wall of the recess and are screwed into an end face of the crankshaft to thereby couple the flywheel to the crankshaft, and the flywheel and the rotor shaft are coupled to each other in a state that the protrusion is fit in the recess.