To provide a practical power generator for supplying electric power to apparatuses disposed in a wheel such as a tire inner pressure alarm device, and to solve a problem caused by early exhaust of the battery in a system using the apparatuses.

The power generator 1 comprises a power generation coil body 2K comprising a plurality of electromagnetic coils 2 disposed on a circumference, and a magnet body 3K comprising a plurality of magnets 3 arranged on a circumference about an axial center CA common to that of the power generation coil body 2K. The magnet body 3K generates a magnetic field varying circumferentially about the common axis CA and is disposed relatively rotatably with the generation coil body 2K. Each electromagnetic coil is disposed at an attitude for generating electromotive force across the magnetic field under the relative rotation of the power generation coil body 2K. The power generation coil body 2K is disposed in the wheel W rotating about the axle A.