The Japanese Marsh Warbler Locustella pryeri is an endangered species in Japan. The main reason probably is the lack of suitable breeding habitats. To protect this species, we must understand what kind of habitat they use in the breeding season. Here, I surveyed their detail breeding habitat preferences at Hotokenuma wetland in Aomori Prefecture. The study was conducted at a much finer scale and over a much longer period than in previous published studies. The results show that in May, territorial males preferred areas where the old dead stems of reed Phragmites australis remained unburnt, whereas from June to July, they preferred areas covered with new reed stems of less than 2 meters in height irrespective of the presence of the old reeds. They also preferred areas covered with dense bushes and areas where water depth was less than about 10 centimeters. This species selects this narrow spectrum of vegetation, and such habitat seems to be limited in Japan. To protect this species, we have to maintain the present breeding habitats and attempt to create new habitats.