An underwater power supply system includes: a working apparatus arranged underwater with at least one power receiving pad; a first battery unit detachably attached to the apparatus with a power supplying pad and battery, the pad configured to supply electric power to the power receiving pad in a non-contact state, the battery electrically connected to the power supplying pad; and an underwater sailing body configured to shuttle between the apparatus and a surface ship or an underwater station suspended from the surface ship, the body configured to carry a second battery unit to the apparatus, detach the first battery unit from the apparatus, and attach the second battery unit to the apparatus, the second battery unit including a power supplying pad and battery, the pad configured to supply the electric power to the power receiving pad in a non-contact state, the battery electrically connected to the power supplying pad.