The JEM-EUSO instrument is a wide-angle refractive telescope in near-ultraviolet wavelength region being proposed for attachment to the Japanese Experiment Module onboard ISS. The instrument will study the fluorescence light produced in atmosphere by UHECR of energy E > 5 1019 eV. It consists of high transmittance optical Fresnel lenses with a diameter of 2.5 m, a focal surface covered by 4932 MAPMTs of 64 pixels, front-end readout, trigger and system electronics. The tracks generated by the Extensive Air Showers produced by UHE primaries propagating in the atmosphere, are reconstructed on the focal surface by registering in a cyclic memory, every 2.5 microseconds, the data coming from the 315648 pixels and by selectively retrieving only th...