A record having a fixed length data portion and a variable length data portion is stored in memory units, each capable of being inputted/outputted independently, in unit of sub-record while the fixed length data portion is divided into a plurality of fixed length sub-records each having the same length and the variable length data portion is made one variable length sub-record. When data operation of the record such as retrieval, update and deletion is required, sub-records necessary for the data operation are limited and the data operation is made in a unit of one or more sub-records, so that data processing of unnecessary sub-records is not made.