LATERAL MODEL, a simulation model of surcharged flow in urban sewer pipe systems, is presented. A number of lateral pipes are connected to the top of sewer pipe in order to drain storm and sewage water from inlet to the sewer pipe. A pressure-control effect that is caused by movement of stormwater into or out of the lateral pipes and has been ignored in overall traditinal simulation models, is taken into account. An equation for determining a pressure wave celerity of the surcharged flow is derived. Run-off experiments are made and the adaptabilities of the model is investigated comparing the experimental results with the simulated ones.