  The Earth's magnetic field effect on the azimuthal distribution of extensive
air showers (EAS) of cosmic rays has been evaluated using a bulk of the Yakutsk
array data. The uniform azimuthal distribution of the EAS event rate is
rejected at the significance level 10^(-14). Amplitude of the first harmonics
of observed distribution depends on zenith angle as A1=0.2*sin^2(theta) and is
almost independent of the primary energy; the phase coincides with the magnetic
meridian. Basing upon the value of measured effect, the correction factor has
been derived for the particle density depending on a geomagnetic parameter of a
shower.
