The elasto-plastic finite element method based on the modified Cam-clay theory was used in numerical experiments to investigate the settlement behavior of an improved ground where vertical drains had been installed in a portion of the ground area. The numerical experiments were performed taking into account the effect of changes in the loading width and the width of the area improved by vertical drains. As a result, the authors have clarified the following facts : the rate of settlement of a ground which is partially improved by installation of vertical drains lags behind the rate predicted by design theory ; the degree of lag becomes more noticeable in the areas closer to the edges' of the improved area and in the later stage of consolidation due to the influence of the adjacent non-improved area. In order for any ground improvement work to be completely effective within a limited period of time, it is necessary to install drains over an area somewhat larger than the designated improvement area. In this paper, a method for the determination of the size of the additional drain-installation area required to properly complete the improvement process is proposed using the results of the numerical experiments.