PURPOSE: To provide a piston-loaded electric power generator which can be suitably used in a telemetry system.

CONSTITUTION: A piston-loaded magnet unit 1 of a piston-loaded electric power generator includes a coil 2 having a permanent magnet core 3 with which magnetic pole extension parts 5, 6 are detained. The unit 1 is mounted on a piston skirt 8, so that these extension parts have a narrow gap from the internal wall 10 of a piston cylinder. The wall 10 is provided with shallow grooves 14 having a crescent section in the longitudinal direction and stretching at the wall periphery for a distance slightly longer than the width of the longitudinal section of the magnetic pole extension parts, when viewed on the section. The width of each groove 14 is equal to the spacing of the extension parts 5, 6, and the spacing in the axial direction of two adjoining grooves 14 is equal to the thickness of the extension parts. With these open grooves in the described arrangement, an electric power is generated for approx. 50% of each turn, in the form of a voltage waveform modulated with frequency and amplitude, and the waveform is rectified, smoothened and regulated, and thus a stable DC level for driving a transducer and telemetry module is established.