1. A programmable controller comprising: a main body unit containing a microprocessor and various memories, the microprocessor operating in accordance with contents of a program memory in which a sequence program is stored and contents of a system memory in which a system program is stored to thereby generate a signal to an output device corresponding to a signal from an input device; and a plurality of input/output extension blocks connected to said main body unit, wherein the programmable controller comprises: block type identifying means that is provided in each of said input/output extension blocks and identifies a type and an arrangement of an input/output extension block which indicate the number of input and output points in said input/output extension block; communication identifying means that is provided in each of said input/output extension blocks and identifies an input/output extension block communicating with said microprocessor; present organization storing means provided in said main body unit, which sequentially reads addresses of the input/output extension blocks with which communication is currently being conducted based on a result of identification by said communication identifying means, and stores input and output numbers corresponding to types and arrangements of input/output extension blocks obtained by said block type identifying means of the input/output extension blocks; and rewrite setting means that determines existence of an organization update instructing signal that is inputted from the outside following change, addition or removal of said input/output extension block and updates, in response to a result of the determination, contents of said present organization storing means by an allocation method selected from a plurality of predetermined allocation methods designated by said organization update instructing signal; and wherein allocation of input/output numbers on said sequence program is carried out by a method selected from said plurality of allocation methods according to contents of said rewrite setting means.