When an information recording and reproducing apparatus records pieces of user data on a write-once optical disk on which information can additionally be written, the information recording and reproducing apparatus performs a closing process so that the pieces of user data can also be reproduced with the reproduction-only apparatus. The closing process includes recording final recording position information indicating an end position of the recorded information in the outside of the user data. Then, in the case where user data is additionally written, the final recording position information is erased using a predetermined pattern through the closing process, and the new final recording position information is recorded in a rear position of the user data.