The KLYPVE optical detector on board the Russian segment of the ISS will observe 10 thousand km2of the Earth atmosphere registering the extremely high energy cosmic ray (EHECR) particles producing fluorescent tracks in the atmosphere. In this article the design of the detector is presented, including: the Fresnel type mirror of 10 m2area, the PMT retina of 2500 pixels, the pixel electronics, the data acquisition electronics, and the trigger system. The detector design is suited to conditions of the space experiment (wide range of temperature, short day-night cycle etc). The problem of selection of rare EHECR events in the presence of high intensity background light is discussed. ©2001 American Institute of Physics.