A logical name of shared data is defined. A network system comprises a communication portion for exchanges of data with other control apparatuses; a link memory (15) for storing data shared through the network; an indirect reference table (16) for storing logical names and the pointers used to access data stored in the link memory; and a database (17) of variables in which logical names are associated with memory addresses in the indirect reference table storing the pointers of the logical names. A control execution portion (18) accesses the link memory through the indirect reference table based on a logical name so that there is no need to know the specific memory location. Specifically, the user program contains logical names and no addresses (pointers) storing data.