PURPOSE: To improve the communication time efficiency by devising a reception section of a master station MODEM so as to discriminate whether a signal sent from a slave station MODEM is a sinusoidal wave signal or a modulation signal.

CONSTITUTION: A sinusoidal wave discrimination section 10 of a equipment section of a MODEM(MD) 1 discriminates whether an incoming signal is a modulation wave such as a training or data signal or a sinusoidal wave for routine reply in response to a signal sent from a transmission section of a MD 2, and in the case of a modulation wave, a data demodulated by a demodulation section 12 is sent to a central processing unit(CPU) 3. When the signal is discriminated to be a sinusoidal wave, the sinusoidal wave is sent to a sinusoidal wave frequency discrimination section 11, where the frequency is discriminated and a signal representing the result of discrimination is given to a data generating section 13 to generate a routine reply data assigned with the frequency and the result is sent to the CPU 3. Thus, the return time at the routine reply time is reduced and the communication time efficiency is improved.