PURPOSE: To realize a small-sized and inexpensive transmission device by uniting a network control device and an MODEM to one body, and executing a rationalization functionally.

CONSTITUTION: In case a data terminal equipment 36 is capable of data communication, if a manual/automatic transmission switching jumper 37 is short, a control part 25 supervises whether a calling signal from the other party exists or not, and whether the telephone concerned is being used or not, through detecting parts 22, 23, and unless both of them are executed, the circuit is connected by operating a circuit connecting relay 20 about 3sec later. When a dial start button switch 29 is turned on, an auto-dial part 27 is operated, a dial pulse sender relay 21 is operated by outputting automatically a set dial number, it is sent out to a telephone circuit 17 through relay contacts 20a, 20b and a switch box 19, and the other party is called. When the other party answers, the fact that the data communication can be executed through the control part 25 is informed to the data terminal equipment 36 through an interface part 34, and a data is transmitted and received to and from the other party.