To prevent diversion to electric apparatuses used in applications other than primary ones by prohibiting power supply to an electric apparatus and warning an user of incompetence if an electric apparatus discriminating means discriminates the electric apparatus as an incompetent one for power supply unit.

An electric apparatus discriminating means 5 involves a control signal detection means 7 which discriminates a control signal to be sent from an electric apparatus 9 connected to a power supply unit, and an electric wire 8 for supplying power to the electric apparatus 9. A power supply prohibit means 6 controls switching of a switching element on the output side by inputting a stopping signal and a constant voltage control signal from a control signal detection means 7. When the electric apparatus 9, in which a microcomputer is built, is operated, the clock signal of the microcomputer or a signal component having a dividing ratio of integral times as much as the clock signal becomes normal mode noise and flows into the power supply unit. By utilizing a principle that a noise component including the clock component has inherent frequency for each electric apparatus, the noise component is detected selectively by a discriminating circuit to discriminate if an electric apparatus to be used is competent.