The layered chalcogenide Ta2NiSe5 has recently attracted much interest as a strong candidate for a long-sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure (P), it is important to clarify the P evolution of a microscopic electronic state in Ta2NiSe5. Here we report the optical conductivity [σ (ω)] of Ta2NiSe5 measured at high P to 10 GPa and at low temperatures to 8 K. With cooling at P = 0, σ (ω) develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV as reported previously. With increasing P, the energy gap becomes narrower and the excitonic peak is diminished. Above a structural transition at Ps ≃ 3 GPa, the energy gap becomes partially filled, indicating that Ta2NiSe5 is a semimetal after the EI state is suppressed by P. At higher P, σ (ω) exhibits metallic characteristics with no energy gap. The detailed P evolution of the energy gap and σ (ω) is presented, and discussed mainly in terms of a weakening of excitonic correlation with P
