  We studied possible evolution of the rotational period and the magnetic field
of the X-ray source RX J0720.4-3125 assuming this source to be an isolated
neutron star accreting from the interstellar medium. Magnetic field of the
source is estimated to be $10^6 - 10^9$ Gs (most probably $\approx 2\cdot 10^8$
Gs), and it is difficult to explain the observable rotational period 8.38 s
without invoking hypothesis of the magnetic field decay. We used the model of
ohmic decay of the crustal magnetic field. The estimates of the accretion rate
($10^{-14} - 10^{-16} M_\odot/yr$), velocity of the source relative to the
interstellar medium ($10 - 50 $ km/s), the neutron star age ($2\cdot 10^9 -
10^{10}$ yrs) are obtained.
