Although inflow/infiltration (I/I) is one of the main factors used to decrease efficiency in sewer capacity and wastewater treatment, due to budget constraints, it is sampled only at the outlet point. Accordingly, the rehabilitation orders of defected sewers are determined for each drainage area without the consideration of I/I in each sewer. This study focuses on the development of a sampled I/I distribution (I2D) model and a decision-making model for sewer rehabilitation. The I2D model is developed based on the unit sewer defect value. The decision making model, developed using a genetic algorithm, is simulated with the consideration of sewer rehabilitation, wastewater treatment cost, and distributed I/I amount in each sewer on the budget. When the simulation results are compared with those of a numerical weighting method, it is concluded that the integrated model provides a detailed and systematic sewer rehabilitation plan from watershed-scale drainage areas to field-scale drainage areas.