1. A data transmission system for transmitting a plurality of numerical codes, successively, comprising: first means for converting each of the numerical codes to be transmitted to a corresponding one of permutation codes represented by a corresponding one of permutations, in time sequence, of a predetermined number of different frequency signals; said first means including matrix means having memories for storing any one of said permutation codes in the form of a matrix, the contents of said memories representing, respectively, elements constituting said matrix so as to indicate the time sequence in transmission of said frequency signals of said one permutation code, means for storing a plurality of weight constants predetermined according to the number of bits constituting each of said numerical codes, means for calculating numerical values by processing said numerical code with respect to said weight constants, successively in order from a larger one to a smaller one of said weight constants, each of said numerical values representing the number of units of the corresponding weight constant contained in a residue of the numerical code after processing of the same with respect to the weight constants larger than said corresponding weight constant, and means for applying a specific one of said permutation codes to said matrix means and effecting shifting of the contents of selected memories in a predetermined mode, repeatedly, by a number of times whose number is dependent on said numerical values, thereby to obtain a new matrix indicative of the time sequence of said frequency signals corresponding to the permutation code of each numerical code, second means for transmitting said different frequency signals in time sequence indicated by said new matrix, and third means for receiving said frequency signals transmitted by said second means and converting said signals into numerical codes.