  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 interstellar medium. Magnetic field of the source is
estimated to be $10^6 - 10^9$ G, and it is difficult to explain observed
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 accretion rate ($10^{-14} - 10^{-16} M_\odot/yr$), velocity of the
source relative to interstellar medium ($10 - 50 $ km/s), neutron star age
($2\cdot 10^9 - 10^{10}$ yrs) are obtained.
