PURPOSE: To reduce the cost and to increases the treating speed in a data conversion system by providing a parameter preparation program used for production and updating of a conversion parameter table and a data form conversion program used for conversion of data of an input data form into the data of a common data form and for output of these converted data.

CONSTITUTION: A parameter preparation program 11 inputs the parameters corresponding to plural input data forms 1 - n through a keyboard 13 and produces and updates a conversion parameter table 12. Meanwhile a data form conversion program 14 inputs the data 15-1 - 15-n having the input data forms 1 - n and converts these data into the data 16 of a common data form by means of the parameter of the table 12 and outputs the data 16. Thus it is possible to obtain the data of a common data form without preparing or updating a conversion program at each time. Then the cost is reduced and the treating speed is increased in a data conversion system.