To provide a self-excited type power unit capable of obtaining a stable operation, even at the time of a low temp. and a light load by using a compact switching element such as an electric field effect transistor(FET).

This power unit is provided with a transformer 3 for boosting a power source 1 and constituted, so that the primary side of the transformer, an oscillation control switching element (for instance, an electric field effect transistor) 4 are connected to the power source in parallel and the secondary side is connected to a main capacitor 11 through a rectifier element 10. In this power unit, a current-voltage conversion means 9 for outputting a voltage signal corresponding to a current flowing to the main capacitor from the secondary side of the transformer is provided, and the on and off of the switching element are controlled according to the voltage signal from the current-voltage conversion means, so as to charge the main capacitor.