A modem (modulator-demodulator) apparatus provided with modem circuits having different signal processing standards, which is optimized for a facsimile machine. Switching of modem functions can be accurately performed by concurrently operating modems with required standards selected from modems with different standards incorporated in a modem apparatus and incorporating in a common input/output port means for selectively switching an input/output of each modem and a dedicated signal detection means required for the switching, and further by constituting a facsimile using the this modem apparatus so as to perform the switching control from the facsimile body or automatically. Moreover, the modem signal after switching can be obtained by using a low speed modem as dedicated signal detection means, detecting a flag pattern after having completed an initial sequence consisting of detection of the presence (power) of an input signal, automatic gain control (AGC), carrier reproduction (PLL), timing synchronization, etc. and deciding that a signal for receiving a low speed modem signal has been received on the condition that at least one of the flag pattern has been detected.