The present disclosure is an aerobic granule forming method using a semibatch-type reaction tank (10) for forming aerobic granules, the aerobic granule forming method being characterized in that: the organic substance includes an easily decomposable organic substance and a slowly decomposable organic substance; and the time of the biological treatment process is adjusted so that the value obtained by multiplying the ratio of the MLSS concentration in the semibatch-type reaction tank (10) to the BOD loading volume of the easily decomposable organic substance in the semibatch-type reaction tank (10) by [time of the operating cycle/time of the biological treatment process] is in the range of 0.05-0.25 kg BOD/kg MLSS/day.