a lower structure adapted to be driven by a travel motor;

an upper structure that is rotatable on the lower structure by a swing motor generator, and

a work equipment mounted on the upper structure and comprising a boom, a stick, and a bucket that are sequentially connected and adapted to be pivoted by a boom cylinder, a stick cylinder and a bucket cylinder respectively;

the work machine further including:

a hybrid type drive system comprising:

an engine,

a motor generator adapted to be driven by the engine so as to function as a generator as well as receive electric power so as to function as an electric motor,

an electric power storage device that serves to store electric power fed from the motor generator functioning as a generator, as well as feed electric power to the motor generator functioning as an electric motor, and

a main pump adapted to be driven either one of or both the engine and the motor generator;

a travel/stick/bucket control circuit that serves to control hydraulic fluid fed from the main pump of the hybrid type drive system to the travel motor, the stick cylinder, and the bucket cylinder;

a boom control circuit that includes a boom pump, which is provided separately from the main pump of the hybrid type drive system, the boom control circuit serving to control hydraulic fluid fed from the boom pump to the boom cylinder; and

a swing control circuit that serves to:

feed electric power from the electric power storage device of the hybrid type drive system to the swing motor generator so that the swing motor generator functions as an electric motor, and

recover to the electric power storage device electric power generated by the swing motor generator functioning as a generator during braking of rotating motion of the upper structure;

wherein the boom control circuit further includes:

an energy recovery motor provided in a return fluid passage through which return fluid discharged from the boom cylinder flows,

a boom motor generator adapted to be driven by the energy recovery motor so as to function as a generator for feeding electric power to the electric power storage device of the hybrid type drive system as well as be driven by electric power fed from the electric power storage device so as to function as an electric motor, and

a clutch that serves to transmit electric power from the boom motor generator functioning as an electric motor to the boom pump and disengage the boom motor generator functioning as a generator from the boom pump.