This paper analyzes survey data from a sample of Japanese collected as part of an international comparative project to test the net cost hypothesis of voluntary behavior. This hypothesis suggests that the higher the net cost of a person's actions, the more he/she will be considered a "volunteer." A questionnaire consisting of 50 questions was collected from a sample of 770 respondents. Differences in the average scores for some of the pairs of activities with different net costs were found to be statistically significant. We also found that the net cost hypothesis is useful in explaining Japanese definitions of volunteering.