  The magnetic phase diagram in a single crystal of copper metaborate CuB_2O_4
in a magnetic field parallel to a tetragonal axis $c$ has been investigated.
  From the resonant, magnetic and magnetostrictive data the phase diagram of
CuB_2O_4 on a plane ``temperature - magnetic field'' is constructed. The
magnetic incommensurate-commensurate phase transition is caused by the
saturation of weak subsystem of copper ions in the strong magnetic field
$H\|c$.
