PURPOSE: To perform signal ciphering undecipherable for a third person without damaging original signals by directly ciphering input signals.

CONSTITUTION: A cipher code 1100 is directly mixed into the input signal 1000 and a secret signal 1200 is obtained. To the input signal 1000, the secret signal 1200 is maintained as it is when the cipher code 1100 is '1' and is inverted when the cipher code 1100 is '-1.' That is, the secret signal 1200 is inverted in sections B1400 and D1600 and as a result, for the ciphered signals, a frequency band is widened and a noise proofing property is improved. The cipher code 1100 is calculated with the code of a certain series (code value: 1, -1) as the cipher code and infinite combinations are present for the code depending on a calculation method. Thus, since the pattern of a code string used for the cipher code 1100 can be inifinitely generated, the signals undecipherable for the third person are generated. On a reception side, when the same cipher code is mixed in again, the original signals are deciphered and the improved signals are obtained.