An encryption technique in which a transmission device and a reception device use solutions generated such that those generated in the same order are assumed to be the same is improved so as to enhance versatility without undermining security. An initial solution respectively used by two communication devices involved in communication in order to generate solutions is sent from one communicating device to the other. Both communication devices generate a mutually agreed-upon number of solutions from the initial solution and set the last solution among the generated solutions as a new initial solution, and using solutions generated based on the new initial solution, the transmission device performs encryption while the reception device performs decryption.