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.
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