This paper considers the ultimate strength of stiffened rectangular plates under compressive and lateral pressure. The approximate analytical method consisted of two parts. The first is elastic large deformation analysis which takes into account the plastification of the stiffener in a simplified way. The second is plastic analysis in which a collapse mechanism is assumed. In the author's first reporto, the plastic analysis was carried out without plastic moment of the stiffener. Thus, the method was too conservative when lateral pressure is predominant. In this report, particular attention is given to the efficiency of plates stiffened on one side and subjected to lateral pressure. For this purpose, plastic analysis is developed from the plastic moment of plate and stiffener considering Faulkner's effective width formula. The analysis and experiments were carried out for various stiffened plates subjected to compression loading or subjected to combination with compression and lateral pressure loading. The following conclusions were obtained : 1) The calculated ultimate strength of stiffened rectangular plates agrees well with the experimental results. 2) The present method provides an effective means of analysing the ultimate strength of stiffened plates under uniform compression and simultaneous lateral pressure.