  The magnetic field dependences of birefringence in a $NdFe_3(BO_3)_4$ single
crystal have been measured in the case where the direction of light propagation
coincides with the trigonal crystal axis $C_3$ ($k\|C_3$), and the external
magnetic field is oriented along the second-order axis $C_2$ ($H\|C_2$). In the
temperature range, in which an incommensurate phase exists with the formation
of a long-period antiferromagnetic helix, the strongly pronounced jumps in the
field dependence of birefringence are revealed and identified as a firstorder
spin-orientation phase transition. The phase transition was accompanied by a
hysteresis in the field dependences of birefringence. The $H-T$ phase diagram
for a $NdFe_3(BO_3)_4$ single crystal has been plotted in the case where the
magnetic field is oriented along the crystal axis $C_2$ ($H\|C_2$).
