A hybrid work machine, comprising: an engine configured to drive a hydraulic pump as a hydraulic source of a hydraulic actuator; an engine-assist motor generator attached to the engine; an electric motor for use in a part of a driving system of the work machine; an electricity storage device configured to store generated power of the motor generator; a power converter disposed between the electric motor and DC buses (80, 81, 80a, 81a); characterised by a buck-boost converter (40, 40a) disposed between the DC buses (80, 81, 80a, 81a) and the electricity storage device, and configured to perform power transfer between the DC buses (80, 81, 80a, 81a) and the electricity storage device by raising and lowering a voltage of the DC buses (80, 81, 80a, 81a); a controller configured to temporarily raise the voltage of the DC buses (80, 81, 80a, 81a) when a torque required for the electric motor exceeds a torque outputtable at the voltage of the DC buses (80, 81, 80a, 81a) connected to the power converter, the controller comprising: a first calculation unit configured to calculate a request torque of the electric motor; a second calculation unit configured to calculate an outputtable torque of the electric motor from a rotation speed of the electric motor and a voltage of the DC buses (80, 81, 80a, 81a) connected to the electric motor via the power converter; a torque comparison unit configured to compare the request torque calculated by the first calculation unit and the outputtable torque calculated by the second calculation unit; and a voltage rise command unit configured to issue a command to temporarily raise a voltage of the DC buses (80, 81, 80a, 81a), provided that the rotation speed of the electric motor is equal to or more than a threshold value when the request torque is equal to or more than the outputtable torque. an engine configured to drive a hydraulic pump as a hydraulic source of a hydraulic actuator; an engine-assist motor generator attached to the engine; an electric motor for use in a part of a driving system of the work machine; an electricity storage device configured to store generated power of the motor generator; a power converter disposed between the electric motor and DC buses (80, 81, 80a, 81a); characterised by a buck-boost converter (40, 40a) disposed between the DC buses (80, 81, 80a, 81a) and the electricity storage device, and configured to perform power transfer between the DC buses (80, 81, 80a, 81a) and the electricity storage device by raising and lowering a voltage of the DC buses (80, 81, 80a, 81a); a controller configured to temporarily raise the voltage of the DC buses (80, 81, 80a, 81a) when a torque required for the electric motor exceeds a torque outputtable at the voltage of the DC buses (80, 81, 80a, 81a) connected to the power converter, the controller comprising: a first calculation unit configured to calculate a request torque of the electric motor; a second calculation unit configured to calculate an outputtable torque of the electric motor from a rotation speed of the electric motor and a voltage of the DC buses (80, 81, 80a, 81a) connected to the electric motor via the power converter; a torque comparison unit configured to compare the request torque calculated by the first calculation unit and the outputtable torque calculated by the second calculation unit; and a voltage rise command unit configured to issue a command to temporarily raise a voltage of the DC buses (80, 81, 80a, 81a), provided that the rotation speed of the electric motor is equal to or more than a threshold value when the request torque is equal to or more than the outputtable torque. a first calculation unit configured to calculate a request torque of the electric motor; a second calculation unit configured to calculate an outputtable torque of the electric motor from a rotation speed of the electric motor and a voltage of the DC buses (80, 81, 80a, 81a) connected to the electric motor via the power converter; a torque comparison unit configured to compare the request torque calculated by the first calculation unit and the outputtable torque calculated by the second calculation unit; and a voltage rise command unit configured to issue a command to temporarily raise a voltage of the DC buses (80, 81, 80a, 81a), provided that the rotation speed of the electric motor is equal to or more than a threshold value when the request torque is equal to or more than the outputtable torque.