1. A memory database system, which updates data by an application program, comprising: a data storage unit to store a master data; and a shared memory area, including: a first area to load said master data and to only be referred to by said application program; and a second area to store a difference data, which is generated when said application program updates said master data in said first area, and to be referred to and updated by said application program, wherein said master data includes a main data and an index for reference for referring to the main data, and wherein said difference data includes an index for update, which links to said index for reference and refers to said difference data.