To provide a traveling body with the improved energy efficiency in starting a propulsion engine or in the air-conditioning system in an aircraft.

An auxiliary power unit of the aircraft as the traveling body comprises a power source unit comprising a fuel cell 11 and a battery 14, and a generator and a motor 7A and 7B having the function of the motor-cum- generator are mounted on main engines 3A and 3B as the propulsion engine. The conversion efficiency of the fuel cell 11 is improved when the power generation efficiency is ≥60%. The electric power can be used for starting the main engine, and the rotational energy can be converted in the electric energy and stored in the battery 14 when the main engines 3A and 3B are stopped, and the power generation of the fuel cell 11 can be minimized.