A virtual intermediate data is prepared between the input data and the
output data by an operator himself. The operator defines a first
conversion rule for converting the input data into the intermediate data
and a second conversion rule for converting the intermediate data into
the output data and derives a conversion rule for directly converting the
input data into the output data on the basis of the first and second
conversion rules. This makes it easy to define and maintain the data
conversion rule for transmitting the data among a plurality of systems
and further improve the processing efficiency of the conversion.