Unit conversion of data in a database is performed, so that efficient use of data can be achieved. VL and VNo in an index associated with each field of a table, define correspondence between a record number and a field value. In the n-th entry of VNo, a rank of the entry in VL is registered where the field value associated with the record having n-th record number in the table is registered. When the system of units of the value registered in VL is not a predetermined system of unit, VLC is provided, so as to apply unit conversion to the value in each entry of VL and to register the converted value in the entry on the same rank in VLC. Then, instead of VL, VCL and VNo define the correspondence between the record number and the field value.