An apparatus for treating a substrate includes a process chamber having a treatment space therein, a support unit that supports the substrate in the treatment space, a gas supply unit that supplies a process gas into the treatment space, an RF power supply that supplies an RF signal to excite the process gas into plasma, and a matching circuit connected between the RF power supply and the process chamber. The matching circuit includes an impedance matching device that performs impedance matching and a harmonic removal device that removes harmonics caused by the RF power supply. The matching circuit operates in a first mode when the harmonics caused by the RF power supply are sensed and in a second mode when the harmonics caused by the RF power supply are not sensed.