To provide a power unit which can be improved in reliability by preventing the deterioration in durability of the unit caused by misuse and, at the same time, can prevent the failure, etc., of an electric device.

A power unit is provided with a voltage-converting means 1 which converts an input voltage into an output voltage by rectifying the input voltage, an output voltage-detecting means 3, incorporated with a comparing means 2 which acts as a feedback circuit for detecting the output circuit, and at the same time, compares the output voltage with a reference voltage, and a constant-voltage control means 4 which controls the output voltage, to suppress the deviation between the output voltage and the reference voltage, based on the compared results of the comparing means 2. The power unit is also provided with an electric device-discriminating means 5 which discriminates whether an electric device is permitted to operate and a power supply stopping means 6 which stops the power supply to the electric device, when the discriminating means 5 discriminates that the device is not permitted to operate.