An aqueous sol containing silica-based particles which sol has a specific surface area of at least 115 m2g aqueous sol and a pH of at least 10. The microcomputer obtains the return light quantity data in the data obtaining period in which the return light quantity is not affected by the prepit, and executes control including the recording power control based on the return light quantity data. Then, the server dynamically generates a metafile, whereby said metafile contains information about the identification, location and format of the media file, and returns it back to the client computer.