This is a computer-readable portable storage medium which is used by a computer managing a plurality of frame buffers and which stores a program enabling the computer to execute a process, and the process comprises preparing an area in which data of a valid chain indicating a connection among frame buffers storing valid image data of the plurality of frame buffers and data of a vacant chain indicating a connection among frame buffers storing no valid image data, is stored, on memory and generating/updating data of the valid chain and the vacant chain when valid image data is stored in one of the plurality of frame buffers and storing it in the memory.