PURPOSE: To make it unnecessary to be conscious of program description to be required for the use of data by storing format information relating to data structure in a file different from a data file.

CONSTITUTION: A data output instruction execution processing part 2 converts data into a format described in a user program 1, outputs the converted result to a data storage device 3 and transfers information relating to data definition information and the format to a data format edition output processing part 4. The processing part 4 edits the information received from the processing part 2 and a data format conversion processing part 6 inputs information relating to the format from a data format storage device 5 and forms an instruction for converting data inputted from the device 3 into an internal state and a data instruction execution processing part 7 executes the received data state converting instruction. The edited data are inputted to a user program 8. Consequently, data structure for data input/output can utilize the information of an external file independently of the existence of formats and the formation/ modification of a data format conversion program can be made unnecessary.