To solve the issues of decreasing revenue of waterworks caused by population decline and increasing renewal cost caused by ageing facilities, it is necessary to strengthen the financial base of the waterworks by undertaking efficient reconstruction of water supply system, and the cost of future renewal demands and anti-seismic upgrading need to be reduced. This paper evaluates the effectiveness of the reconstruction of water supply system from the economic viewpoint and identifies affecting factors. Assessments show it is more effective to terminate 60% of plants. The following factors were taken into account; intake water amount, length of conveyance pipe, diameter of conveyance pipe, difference in elevation between the abolished and integrated plants, and ratio of hourly maximum amount of water distribution, treatment capacity ratio, the anti-seismic capacity of abolished plants. Out of these factors, the intake water amount from abolished plant to merged plant has the highest correlated factor affecting the reconstruction of water supply system from the economic viewpoint.