Image data encoded in units of a tile is processed to define a hierarchical structure by repeatedly forming one tile group from adjacent tiles and another tile group from adjacent tile groups. Plural partially encoded data a tile located at a terminal of the hierarchical structure are arranged in ascending order of priority of decryption to define a tree having as nodes tile groups, tiles, and partially encoded data. Encryption key information is generated for a node located at an uppermost layer of the tree and, up to a node located at a terminal, encryption key information is generated for a node of interest based on encryption key information for a node at an upper layer. When a node of partially encoded data is designated as an object to be encrypted, partially encoded data at a higher layer containing the designated object is set as an object to be encrypted.