Plasma impedance monitoring in semiconductor manufacturing processes is performed using external sensors, such as voltage-current (VI) probes or directional couplers. Plasma chamber impedance measurements, conducted in non-50 Ω matched transmission lines, suffer from a lack of clean signals due to phase variations and the nonlinearity of plasma, thus, sensor calibration is required for each installment. In this study, we monitored plasma impedance in situ based on the position of the vacuum variable capacitor within the matching network, without employing an external VI probe. We observed changes in the matching position according to parameter variations and subsequently confirmed that the calculated plasma impedance also varied accordingly. This study demonstrates the feasibility of real-time plasma impedance monitoring under 50 Ω-matched conditions without the use of external sensors, thereby simplifying plasma diagnostics.