Recorded at high density on an optical disk are at least one title set and a volume information file. The title set consists of a plurality of files aligned on a boundary between logical sectors. The files store title set management information and data objects to be reproduced. To retrieve data from the optical disk, volume file information is read out, thereby acquiring information about the desired title set. Then, the information for indicating steps or reproducing the data object and managing the desired title set is acquired. Using the title set management information, data is reproduced from the optical disk.