1. A data conversion system including a processor and a memory device storing instructions executable by said processor for converting data transferred between a first data processing system and a second data processing system, which systems operate cooperatively in a configuration of a complex system including a plurality of data processing systems, comprising: means for accessing a first conversion rule table, the first conversion rule table storing a data format for each of the data processing systems, an intermediate data format which is a data representation format to be commonly used for all the data processing systems, and an input/output type for indicating a direction of conversion, so as to have a correspondence relationship therebetween; means for determining a first data format conversion rule using information identifying the first system, the first data conversion rule being a rule for converting first data into first intermediate data, the first data being expressed by a first data format which is a data expression format specific for the first system, the first data having a first meaning, the first intermediate data being expressed by an intermediate data format which is a data expression format, the first intermediate data having the first meaning of the first data but being in the intermediate data format; means for determining a data meaning conversion rule using identifier information of the data meaning conversion rule, the data meaning conversion rule being a rule for converting the first intermediate data into second intermediate data, the second intermediate data having a second meaning but being expressed in the same intermediate data format as the first intermediate data; means for determining a second data format conversion rule using information identifying the second system, the second data format conversion rule being a rule for converting the second intermediate data into second data, the second data being expressed by a second data format which is a data expression format specific for the second system, the second data having the second meaning, the data expression format specific for the second system being different from the data expression format specific for the first system; means for converting the first data into the second data based on the determined first data format conversion rule, the determined data meaning conversion rule, and the determined second data format conversion rule; means for outputting the converted second data for the second data processing system; and means for enabling multiple users to concurrently update definitions used for any of the first data format conversion rule, the data meaning conversion rule, and the second data format conversion rule, wherein, in each of the first data format conversion rule and the second data format conversion rule, data before conversion and data after conversion have identical meaning, wherein the first data format conversion rule is a conversion rule of converting physical data into common data, wherein the data meaning conversion rule is a conversion rule of converting common data into common data, wherein the second data format conversion rule is a conversion rule of converting common data into physical data, and wherein the first data format conversion rule, the data meaning conversion rule, and the second data format conversion rule are defined by referring to information of conversion rules between physical data and common data and information of conversion rules between common data and common data.