To provide a buoy capable of obtaining sufficient power by thermoelectric generation.

The buoy comprises: a chamber 3 provided with a first portion 4 and a second portion 7 each thermally connected to water and atmospheric air, the internal pressure of the chamber 3 being reduced; a thermoelectric element 18 thermally connected inside the chamber 3 to each of the first portion 4 and the second portion 7; first and second rechargeable batteries 21, 22 to which a first power P1 output from the thermoelectric element 18 is charged; a processing unit 30 driven by a second power P2 output from the first rechargeable battery 21; a heating wire 23 disposed inside the chamber 3 and caused to generate heat by a third power P3 output from the second rechargeable battery 22; and a first control unit 31 for charging the second rechargeable battery 22 with the first power P1 when a voltage V₁ of the first rechargeable battery 21 is higher than or equal to a first prescribed voltage V_s1, and stopping supply of the first power P1 to the second rechargeable battery 22 when the voltage V₁ is lower than the first prescribed voltage V_s1.