This research formulates the two-dimensionally model of a membrane with crease line which is always added when the thin material is used for deployable space structure like a solar sail. Recent space development is tending to focus on installing a system on a ultra-light weight structure, so when the membrane is chosen as the base of space structure, the keeping accuracy of the membrane surface is the point of interest. This means how to reduce the wrinkle appeared on the membrane becomes the key point. However, the most research on modelling the membrane limit to the one dimension model, or if any, the plastic deformation is not considered. Here in this paper, the authors established the two-dimensionally membrane model considering the plasticity and clarified from analysis and experiment that wrinkles can be reduced by giving crease to a membrane. In the model assuming the actual antenna installed structure and with the actual load condition, the fact is obtained that a crease perpendicular to the loading direction should be applied to a position one fifth between the load application point and the wrinkle generated region. This position can be considered to be able to reduce wrinkle the most from the view point of amount of energy required to eliminate wrinkles.