The statistical analysis of the characteristic frequency foF2 of the Earth's ionosphere averaged over mid-day hours – from 11:00 till 17:00 h LT – is carried out. Disturbances of foF2 connected to earthquakes are considered on the background of seasonal, geomagnetic, 11-years and 27-days solar variations. A special normalized parameter <i>F</i> is introduced, which represents the almost seasonal-independent part of foF2. Days with high solar (Wolf number >100) and geomagnetic (&sum;<i>K_p</i>>30) activity are excluded from the analysis. Events with magnitude <i>M</i>>5, distance from the sounding station <i>R</i><500 km and depth <i>h</i><70 km are taken into account. The superimposed epoches' method is used to determine the temporal dependence of <i>F</i>. It is found that <i>F</i> increases about 3–6 days before the earthquakes and then decreases one day – two days before the shock. The decreased values of <i>F</i> continue to exist two-three days after events with <i>M</i>>5.5. The obtained phenomenon depends on the magnitude of the earthquake. For events with <i>M</i>>5.5, the reliability of the effect is larger than 95%. For data of more than 80 earthquakes in the vicinity of Petropavlovsk-Kamchatsky and more than 200 earthquakes in the vicinity of Tokyo analogous results are obtained.