  Electrical resistivity and ac-susceptibility measurements under high pressure
were carried out in high-quality single crystals of $\alpha$-Mn. The
pressure-temperature phase diagram consists of an antiferromagnetic ordered
phase (0<$P$<1.4 GPa, $T<T_{\rm N}$), a pressure-induced ordered phase
(1.4<$P$<4.2-4.4 GPa, $T<T_{\rm A}$), and a paramagnetic phase. A significant
increase was observed in the temperature dependence of ac-susceptibility at
$T_{\rm A}$, indicating that the pressure-induced ordered phase has a
spontaneous magnetic moment. Ferrimagnetic order and parasitic ferromagnetism
are proposed as candidates for a possible magnetic structure. At the critical
pressure, where the pressure-induced ordered phase disappears, the temperature
dependence of the resistivity below 10 K is proportional to $T^{5/3}$. This
non-Fermi liquid behavior suggests the presence of pronounced magnetic
fluctuation.
