  We investigate phase transitions of the quasi-one-dimensional XXZ model in a
magnetic field, using the bosonization combined with the mean field treatment
of the inter-chain interaction. We then find that the field induced
incommensurate order is certainly realized in the low field region, while the
transverse staggered order appears in the high field region. On the basis of
the result, we discuss the field-induced phase transition recently observed for
BaCo2V2O8.
