Selection algorithm for combination of positioning satellites to be augmented within the bit rate of L6 signal of QZSS is developed. In semi-real time, time sequence of the combination is planned with criterion of the geometry, and then the combination is determined at each epoch based on the plan and integrity information obtained for each satellite in real time. In simulation concerning GPS, QZSS, GLONASS and Galileo, validity of the algorithm is shown in terms of statistics of DOP in open sky environment in each area of Japan. The algorithm is implemented in Centimeter Level Augmentation Service utilizing four-satellite constellation of Quasi-Zenith Satellite System.