A data compressing/decompressing apparatus is suitable for compressing data containing plural-byte characters, for instance, a Japanese language text. The data compressing/decompressing apparatus owns a homonym dictionary in which KANJI-character idioms, KANJI-character-reading, and homonym discrimination information are stored in correspondence with each other. This data compressing/decompressing apparatus converts a KANJI-character idiom contained in declarative sentence data into phonetic data, and further compresses this phonetic data to output the compressed phonetic data. The phonetic data is such data that this KANJI-character idiom is replaced by information constituted by character number discrimination information indicative of the character number about KANJI-character idiom reading, the reading thereof, and the homonym discrimination information thereof.