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The capability of an on-site sewage treatment system was examined in Nagareyama, a suburb of Metropolitan Tokyo, with the multi-objective optimization method. The total cost, local expense, drinking water quality, stream water quality and speed of sewerage service were considered and the relative weight of each evaluated by interviews with 44 citizens. In the optimal sewerage plan, about half of the planning area was concluded to be suitable for onsite treatment systems. The idea presented here is expected to be useful for other countries especially for developing ones.