To provide a buoy which improves usability without using a dry cell.

The buoy 1 comprises a buoy body 2 floating on a water surface X; and an installation member 3 which is attached to the buoy 2 at an upper part 31 in such a manner that it can move freely in a vertical direction A to locate a lower part 32 undersea Y. The buoy body 2 is provided with a pinion 6; a generator 7 for generating electric power using the revolution of the pinion 6; and a lamp 8 for indicating the position to an outside O using the generated power by the generator 7. The installation member 3 is composed of an arbor 9 the upper part 31 of which is inserted into a case 42 and further into a space 51 of an upright installed part 5; and a circular plate 10 attached horizontally to a bottom of the arbor 9. A rack 12 engaging with the pinion 6 is extended in the vertical direction A to be attached to the upper part 31 of the arbor 9.