A programmable logic controller comprising: a variable table (Tb); a storage device; and an interface circuit, wherein: the programmable logic controller is configured to perform 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 (Tb) 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; the storage device has a memory region (Mn) for storing values of the reference variables; and the interface circuit is configured to mutually partially duplicate communication data for use in communication between the programmable logic controller and the different programmable logic controller, and to mutually partially duplicate the values of the reference variables stored in the memory region (Mn), characterized in that the different programmable logic controller includes an expansion programmable logic controller expected to be additionally installed in the distributed control system; and the programmable logic controller is configured to secure in advance the memory region (Mn) for storing the reference variable, which is communicated between the programmable logic controller and the expansion programmable logic controller in the case where the expansion programmable logic controller is additionally installed in the distributed control system, in the storage device on the basis of the variable table (Tb) to permit additional installation of the expansion programmable logic controller. a variable table (Tb); a storage device; and an interface circuit, wherein: the programmable logic controller is configured to perform 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 (Tb) 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; the storage device has a memory region (Mn) for storing values of the reference variables; and the interface circuit is configured to mutually partially duplicate communication data for use in communication between the programmable logic controller and the different programmable logic controller, and to mutually partially duplicate the values of the reference variables stored in the memory region (Mn), characterized in that the different programmable logic controller includes an expansion programmable logic controller expected to be additionally installed in the distributed control system; and the programmable logic controller is configured to secure in advance the memory region (Mn) for storing the reference variable, which is communicated between the programmable logic controller and the expansion programmable logic controller in the case where the expansion programmable logic controller is additionally installed in the distributed control system, in the storage device on the basis of the variable table (Tb) to permit additional installation of the expansion programmable logic controller. the programmable logic controller is configured to perform 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 (Tb) 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; the storage device has a memory region (Mn) for storing values of the reference variables; and the interface circuit is configured to mutually partially duplicate communication data for use in communication between the programmable logic controller and the different programmable logic controller, and to mutually partially duplicate the values of the reference variables stored in the memory region (Mn), characterized in that the different programmable logic controller includes an expansion programmable logic controller expected to be additionally installed in the distributed control system; and the programmable logic controller is configured to secure in advance the memory region (Mn) for storing the reference variable, which is communicated between the programmable logic controller and the expansion programmable logic controller in the case where the expansion programmable logic controller is additionally installed in the distributed control system, in the storage device on the basis of the variable table (Tb) to permit additional installation of the expansion programmable logic controller.