To provide the data converter and method by which an arithmetic operation is conducted without extending a run length code and to provide the program supply medium that supplies a data conversion program.

A repetitive number storage register 22 receives a run length coded data stream stored in a data storage device 1 via a data input line and stores a repetitive number (C1) in the run length coded data stream. An immediate value storage register 23 stores an immediate value (V1) in the data stream. An arithmetic section 24 applies arithmetic operation to the immediate value (V1) stored in the immediate value storage register 23. An arithmetic result storage register 25 stores the arithmetic result (V0) in the arithmetic section 24. The arithmetic result (V0) stored in the arithmetic result storage register 25 is used for an immediate value and it is fed to a data storage device 2 connecting to, e.g. a data output line 26 in pairs with a repetitive number (C0).