A fundamental algorithm using the relative position and velocity vector and flight intent for the aircraft self-separation operation in a high density air corridor is presented. A high density air corridor is expected to be an air space where aircraft capable of airborne self-separation are allowed to fly in the same direction. An appropriate self-separation algorithm is indispensable to operate it safely and efficiently. In this study, a typical free-flight algorithm is examined to investigate its suitability for the corridor operation. Through a series of traffic simulations, we clarify that the free-flight based algorithm causes many aircraft to perform excessive heading change maneuvers, and frequent conflict occur against pilots' intent. To avoid any conflict, the self-separation algorithm is improved by introducing the flight intent in the corridor that all aircraft intend to fly in the same direction. Through the numerical simulation, the improved algorithm facilitates a more intuitive aircraft maneuver to achieve the conflict-free operation with much fewer maneuvers. It is concluded that the flight intent has a significant role to develop a self-separation algorithm capable of the safe and efficient high density corridor operation.