  Superconductivity under pressure in FeSe ($T_{\rm c}$$\sim$7.5 K) has been
investigated using single-crystal specimens through the measurements of DC
magnetization and electrical resistivity. A characteristic three-step increase
in $T_{\rm c}$ has been found under hydrostatic pressure up to $\sim$34 K above
7 GPa. The structural transition from a tetragonal phase to an orthorhombic
phase ($T_{\rm s}$$\sim$87 K) is found to disappear at $P$$\sim$2.3 GPa, above
which $T_{\rm c}$ increases rapidly, suggesting that the superconductivity is
enhanced by the tetragonal environment. Under non-hydrostatic pressure, the
increase in $T_{\rm c}$ is suppressed and the superconductive volume fraction
is considerably reduced above 2 GPa, probably owing to the breaking of the
tetragonal lattice symmetry by the uniaxial stress. The intimate correlation
between the enhanced (suppressed) superconductivity and the tetragonality
(orthorhombicity) in the phase diagram is a common feature of FeSe and other
iron-pnictide superconductors.
