Abstract: To make window systems which are widely used in personal computers and applications for personal computers operable in a portable information terminal which does not have a sufficient processing capability, input information such as a key input from operator generated in a portable information terminal 210 is sent to a remote supporting server 240 where it executes or simulates an arbitrary window system which is free from restriction of functions to generate a rendering image. The supporting server then converts the rendering image to an image suitable to the portable information terminal and sends it back to the portable information terminal 210 as a rendering instruction. The portable information terminal 210 receives the rendering instruction of this optimized image for display.