  Magnetization measurements under a hydrostatic pressure of P = 1.4 GPa were
performed on the coupled spin dimer system TlCuCl_3, which exhibits a
pressure-induced quantum phase transition from a gapped singlet state to an
antiferromagnetic state at P_c = 0.042 GPa. Antiferromagnetic ordering with
ordered moments parallel to the ac plane was observed at T_N = 16.7 K at 1.4
GPa. The spin reorientation phase transition was observed at T_R ~ 9.2 K for
zero magnetic field, at which the ordered moments start to incline towards the
b axis. With increasing external field parallel to th b axis, a second-order
phase transition from the oblique antiferromagnetic (OAF) phase to the
spin-flop (SF) phase occurs below T_R. We argue that the OAF phase arises from
the competition between the anisotropic energies of the conventional second
order and the fourth order, that increases with increasing pressure. We discuss
the OAF-SF transition within the framework of the mean field approximation.
