  We report an analysis of the effects of magnetic field on a
quasi-one-dimensional band of interacting electrons with a transverse
dimerizing potential. One-particle problem in bond-antibond representation is
solved exactly. The resulting propagator is used to calculate the
spin-density-wave (SDW) response of the interacting system within the matrix
RPA for the SDW susceptibility. We predict the magnetic field induced
transition of the first order between interband SDW_0 and intraband SDW_(+-)
phases. We reproduce the rapid oscillations with a period of 260 Tesla and the
overal profile of the TMTSF_2ClO_4 phase diagram.
