1. A programmable logic controller comprising: a variable table; a storage device; and an interface circuit, wherein: the programmable logic controller performs sequence control in coordination with a different programmable logic controller to constitute a distributed control system; the programmable logic controller includes reference variables which are variables to be referenced by the different programmable logic controller in the sequence control; the variable table includes variable information including identification symbols that indicate individual programmable logic controllers that constitute the distributed control system and variable names of the reference variables, the variable information being set for each of the reference variables; in a case where the distributed control system is expandable to include an additional programmable logic controller and the additional programmable logic controller is not connected to the distributed control system, the variable table includes information for a reference variable that is not connected to the distributed control system; the storage device has a memory region for storing values of the reference variables; and the interface circuit mutually partially duplicates communication data for use in communication between the programmable logic controller and the different programmable logic controller, and mutually partially duplicates the values of the reference variables stored in the memory region.