  The ac susceptibility and de Haas-van Alphen (dHvA) effect in UGe$_2$ are
measured at pressures {\it P} up to 17.7 kbar for the magnetic field {\it B}
parallel to the {\it a} axis, which is the easy axis of magnetization. Two
anomalies are observed at {\it B$_x$}({\it P}) and {\it B}$_m$({\it P}) ({\it
B$_x$} $>$ {\it B}$_m$ at any {\it P}), and the {\it P}-{\it B} phase diagram
is presented. The Fermi surface and quasiparticle mass are found to vary
smoothly with pressure up to 17.7 kbar unless the phase boundary {\it
B$_x$}({\it P}) is crossed. The observed dHvA frequencies may be grouped into
three according to their pressure dependences, which are largely positive,
nearly constant or negative. It is suggested that the quasiparticle mass
moderately increases as the boundary {\it B$_x$}({\it P}) is approached. DHvA
effect measurements are also performed across the boundary at 16.8 kbar.
