PURPOSE: To provide the portable vehicle telephone set in which no large current flows to a fuse even in the event of mis-installation and a blown fuse due to mis-installation is prevented.

CONSTITUTION: This system is provided with a portable vehicle telephone set having a power supply section, an external power supply supplying power to the telephone set, and a hand-free with an adaptor unit built therein and connected between the portable vehicle telephone set and the external power supply, and the adaptor unit is provided with a reverse connection prevention circuit having a switch 24 turning on/off the connection between the power supply section and the external power supply and a diode 28 as a means to restrict flowing of a current from a ground 27 to the telephone set. Thus, even when a power supply is reversely connected, no current flows to the portable telephone set and the hand-free and a blown fuse is prevented.