The present invention provides a power unit of an underwater vehicle, which allows reduction in size and weight of a transformer for stepping down a power voltage of a power supply to increase a loading amount of a variety of devices with respect to the underwater vehicle, so as to obtain a favorable operating environment. The power unit is connected to the power supply through a cable and receives power through the cable. Further, the power unit comprises an electronic transformer having: a rectifier for rectifying a high-voltage alternate current power of the power supply into a direct current; a high-frequency converter for converting an output of the rectifier into a high-frequency alternate current; a transformer for stepping down an output voltage of the high-frequency converter; a rectifier for rectifying an output of the transformer into a direct current; and a control circuit for controlling the high-frequency converter.