2015 年 45 巻 2 号 p. 213-224
Recent low birthrates and an aging population have progressively brought the accumulation of enormous social capital infrastructures to a crisis. To secure a comfortable civic life, we must advance arguments to minimize the expenses for the renewal and maintenance of the infrastructure while maintaining improvements of the service level and providing accountability to citizens.
However, quantitative verification is still lacking and it is difficult to plan and perform the renewal and maintenance of small enterprises such as sewerage systems. That is why it is important that we scientifically show the optimum renewal and maintenance planning from both short-term and long-term viewpoints. Then, we will be able to practice quantitative asset management of institutions as a business entity.
This study aimed to determine the state of sewerage pipe lines from several perspectives and incomes or expenses as the business entity in Nagaokakyo City （Kyoto Prefecture）, and showed a scheme for renewal and maintenance planning of the sewer institution.
First, we determined the present conditions of the sewer business in terms of running costs, user charges, time and the types of sewerage pipe lines. Therefore, there is a need to minimize total social costs （renewal costs, repair costs, amount of damage, and maintenance costs for checking and cleaning） to constantly maintain the solid pipe line network as a business entity on condition that people use an institution comfortably and continually as a service level.
Second, we developed a theory for possible trouble in sewerage pipes as a basic factor for counting social benefits and costs, formulated each cost used to calculate the total social costs, and constructed a model to decide the time for update and a method that minimizes the total social costs within the budget limitations.
Finally, we make suggestions about the priorities for renewal and maintenance planning, and arranged the various problems to decide the priority （such as setting various parameters for the trouble rate and amount of damage per user）. To solve these problems, area cross-sectional and chronological approaches based on the sewer database and questionnaires from users are needed.
JEL Classification:C54, H39, O21, P25, R11
