PURPOSE: To improve the disturbance resisting property of a digital communication system, by relating a hopping pattern of a frequency hopping system using an error correction code with the correcting capability of the error correction code.

CONSTITUTION: Information from an information source is converted into a binary sequence at a binary sequence converter 1 and the output of the converter 1 is inputted into an error correction encoder 2 where a redundancy is added to the output. The output of the encoder 2 is outputted to a communication path 4 after it is modulated and converted into a signal suitable for the communication path 4 at a modulator 3. At the communication path 4, the signal receives disturbance caused by noises, etc., and inputted into a demodulator 5 where the signal is returned to the binary sequence containing the redundancy. The output of the demodulator 5 is corrected in terms of errors by using the redundancy at an error correcting decorder 6 and restored to the binary sequence information, and then, converted into a form suitable to users at an information converter 7. This modulating system adopts a frequency hopping system which avoids disturbance from the cmmunication path 4 by randomly switching the frequency, but a restriction is given to the hopping channel pattern in one block and the number of erroneous bits caused by disturbance in one block is suppressed to ≤3.