The present study investigated the attitude of exercise adherence and aquatic environment in swimming across Lake Kasumigaura. The participants were 60 males and 17 females (23-77 years) who completed a questionnaire survey. The questionnaire was composed of 27 items that represented 6 categories: self-efficacy, exercise-induced feeling, self-esteem, exercise adherence, participation meaning, and aquatic-environmental attitude. The results indicated that the participants had a positive feeling for efficacy of swimming, physical self-perception and exercise adherence, and got a significant concern for aquatic environment. It suggests a possibility that swimming across a lake leads to the change of attitude not only exercise adherence but also preserving nature and enjoying swimming in open water such as a lake.