1. A trajectory planning method for obtaining a trajectory for controlling a state of an object toward a goal state by a trajectory planning system, the method comprising the steps of registering, by a search tree generating section of the trajectory planning system, a state of the object as a root of a search tree in a state space; registering, by the search tree generating section, a next state of the object after a lapse of a predetermined time interval obtained through dynamical relationships during the time interval as a branch of the search tree in the state space; determining, by a known-state registration tree generating section, a cell to which the next state belongs among a plurality of cells which have been previously prepared by segmenting the state space; determining, by the known-state registration tree generating section, whether or not a state which belongs to the cell has already been registered as a branch of a known-state registration tree; discarding the next state when a state which belongs to the cell has been registered; registering the next step as a branch of the known-state registration tree in a known-state registration tree storing section when a state which belongs to the cell has not been registered; selecting, by a trajectory generating section of the trajectory planning system, a state whose distance to the goal state is minimum among states registered as branches of the known-state registration tree; and obtaining, by the trajectory generating section, a trajectory using a sequence of states in a backward direction from the state toward the root of the known-state registration tree.