To low-costly and effectively purify water in a water reservoir system.

The water purification apparatus comprises a convection generation unit 2 allowed to float on a water surface in a predetermined position in a water reservoir system and generating convection by a blade 2c rotated by a drive motor 12a, a solar panel 8, a capacitor 12c which stores an electric current generated by the solar panel 8, and a discharge quantity control circuit 12b that controls the quantity of the discharged electricity so as to supply the current stored in the capacitor 12c by a definite quantity per time. In one embodiment, a support casing 3 that supports the convection generation unit 2, the solar panel 8, the drive motor 12a, the capacitor 12c, and the discharge quantity control circuit 12b has a float 5 connected to its surface and has a protruded engagement means 6 which freely and vertically engages with a guide column 7 uprightly standing on the bottom of water in the closed water area.