PURPOSE: To attain the data transmission even after working hours pass by receiving a prescribed dial signal transmitted from a telephone set and providing, at an accumulating program control swichboard, a connecting means to connect first and second data terminal device through a modulating demodulating device with the output of a register.

CONSTITUTION: Based on the request of the talker of a first data terminal equipment 11, the listener of a telephone set 4 executes a hooking operation. A central processor 3 temporarily holds the calling circuit of the first data terminal device 11 and connects the telephone set 4 to a register 5. The listener of the telephone set 4 dials a number set beforehand and inputs the dial signal to the register 5. When the central processor 3 decodes the dial signal of the register 5 and discriminates that the contents to request the data communication are obtained, outputs a control signal and connects the first data terminal equipment 11 through a MODEM 6 to a second data terminal equipment 7. Thus, even after the working hours pass, the data can be transferred from the first data terminal equipment 11 to the second data terminal equipment 7.