We propose a real-time camera calibration method for an autonomous flight of a small helicopter. Our purpose is to control a small helicopter automatically by using cameras fixed on the ground. We use calibrated cameras, un-calibrated cameras, and the small helicopter that is not attached with any sensors. The proposed method finds correspondences between image features in the two images of a calibrated camera and an un-calibrated camera, and estimates the extrinsic parameters of cameras using a particle filter in real time. We evaluate a utility of the proposed method by experiments. We compare real-time calibration by typical bundle adjustment with a Gauss Newton method to the proposed method in the experiment of the small helicopter flight. The autonomous flight of the small helicopter can be achieved by the proposed method in the situation that a flight of a helicopter can not be achieved with typical bundle adjustment with a Gauss Newton method.