The dependence of the A_p-index describing geomagnetic disturbance on phases of the Moon is studied. The available data for the 20^th—23^rd 11-year activity cycles are processed through the epoch superposition method. For the falling branch of a solar cycle, maximum values of the A_p-index are found to take place near new moon. The difference of the A_p-index values for new moon and full moon is about 18 %. For the phases of increase and maximum of a solar cycle, minimum values of the A_p-index are found to occur during a few days before full moon, and maximum values of the A_p-index take place during a few days after full moon. From this follows the conclusion that the effect of the Moon on the Earth’s magnetosphere differs essentially for the intervals near new moon and full Moon.