Despite the fact that many rivers exist in Yakushima island, Hydrobryum puncticulatum inhabits only in the middle stream of Issoh River. To clarify their habitat conditon and growth process, a multi water quality meter was installed in the middle stream of Issoh River where Hydrobryum puncticulatum was observed. During the summer season, the pH value of daytime showed about 0.4 higher than that of night time, indicating that the daytime H⁺ concentration was 2.5 times higher than the night time H⁺ concentration. Issoh River has a high stream velodity due to its steepish slope. Therefore, photosynthetic species such as algae are difficult to live in Issoh River. The difference in the H⁺ concentration between daytime and night time may be attributed to photosynthetic activity by Hydrobryum Japonicum. Other factors than Hydrobryum puncticulatum habitation could be the water temperature and stream velocity in the upper steream, and salinity in the downstream.