An inexpensive engine assist device capable of performing stable energy regeneration from an accumulator and a working machine on which the engine assist device is mounted are provided. A variable capacity type main pump (7, 8) and a variable capacity type assist pump (16) having a motor function and a pump function are directly connected to an engine (6). A return pressure oil flowing out from a fluid pressure actuator (3a, 9) is temporarily accumulated by a sub-accumulator (18) and is supplied to an inlet of the assist pump (16), and the assist pump (16) pressurizes the return fluid pressure oil and supplies the return fluid pressure oil to a main accumulator (17). A controller calculates and controls an assist pump swash plate angle using an engine load torque, and assist starting torque or charge starting torque set by engine speed setting means, and introduces a pressure-accumulated fluid discharged from the main accumulator (17) to the inlet of the assist pump (16) or introduces a pressurized fluid discharged from an outlet of the assist pump (16) into the main accumulator (17).