  Optical conductivity [$\sigma(\omega)$] of YbS has been measured under
pressure up to 20 GPa. Below 8 GPa, $\sigma(\omega)$ is low since YbS is an
insulator with an energy gap between fully occupied 4$f$ state and unoccupied
conduction ($c$) band. Above 8 GPa, however, $\sigma(\omega)$ increases
dramatically, developing a Drude component due to heavy carriers and
characteristic infrared peaks. It is shown that increasing pressure has caused
an energy overlap and hybridization between the $c$ band and 4$f$ state, thus
driving the initially ionic and insulating YbS into a correlated metal with
heavy carriers.
