When the storage capacity of a conventional optical disk is increased, it has been conventionally impossible to express the increased addresses by using the number of bits used to represent the address information on the optical disk. An address format enabling expression of the increased addresses while maintaining the compatibility with conventional address formats even by using the same number of bits is proposed. The track of an optical disk of the invention is divided into blocks each constituted of L sub-blocks. In each sub-block, first M-bit digital information specifying the block addresses of the sub-blocks and secondN-bit digital information capable of expressing a number not less than L are recorded. If the value of a block address is below a threshold, the second digital information represents values specifying the numbers previously allocated to the sub-blocks; and if the value of a block address is not less than the threshold, the second digital information represents other values than the values specifying the numbers previously allocated to the sub-blocks. Alternatively, if the value of a block address is not less than the threshold, the second digital information represents the values specifying the numbers previously allocated to the sub-blocks; and if the value of a block address is below a threshold, the second digital information represents other values than the values specifying the numbers previously allocated to the sub-blocks.