In this paper, a new mathematical population model to explain a decreasing tendency is constructed. Using this model, we analyze population changing tendencies of self-governing bodies in south part of Tohoku region. The tendency of population decreasing is analyzed into two properties. One is common in the whole region and the other is individual in each self-governing body. By mean of this model, we well were able to explain the population, especially in self-governing bodies which population are decreasing.
