To provide a multi-transmission device and its system that effectively reduce the number of fixed-type connecting ports to be disposed, and effect weight saving of a communication device.

In the multi-transmission device and its system, a signal switching circuit and a related signal transmission line are disposed in a main circuit. The fixed-type connecting port connects the signal switching circuit and the related signal control circuit inside the main circuit system. The signal switching circuit determines a type of a connected external device based on the level of an earthing signal of the connecting port of the external device, switches the prefixed signal transmission lines, and outputs corresponding signals to the external device. The circuit between the external device and the related control circuit is closed, and the body of the device is connected to multiple kinds of external devices from the fixed-type connecting ports.