Surveys of aquatic plants distribution and salinity were conducted using a canoe at Lake Tofutsu, Hichirippu Pond, Mochirippu Pond and Tosamuporo Pond.　All four of them are brackish water lakes in the eastern part of Hokkaido Island.　In Tosamupolo Pond, only Zostera marina was growing.　In Hichirippu Pond and Mochirippu Pond, Z. marina and Z. japonica were distributed.　In addition to these two species, eight aquatic plant species were found in Lake Tofutsu.　In Lake Tofutsu, Z. marina and Z. japonica were distributed as far as 5 km away from the mouth of the lake, and they could not be found anywhere beyond.　Both species appeared mainly in places where the salinity was 1% or more.　The area far from the mouth of Lake Tofustu had salinity of 1% or less, and other species, except the two Zostera species, were present.　However, growth of Z. japonica was also confirmed in area of low salinity.　The salinity of Hichirippu Pond, Mochirippu Pond and Tosamuporo Pond were range from 2.61% to 3.15%, and it became clear that only the two Zostera species were able to grow. It was found that the distribution of aquatic plants in the brackish water lake is closely related to salinity.　This suggests that brackish water lake with high biodiversity has a wide range of salinity within the lake.　Japanese Ministry of the Environment listed 56 lakes as brackish water lakes in Japan, of which 23 lakes exist in Hokkaido.　Although there are past survey data on 21 of the 23 lakes, there has been no survey in recent years.　These facts show the necessities to conduct regular monitoring surveys on brackish water lakes in Hokkaido.