Disclosed is an apparatus for treating a substrate. The apparatus includes a chamber having a space therein in which the substrate is treated, a support unit that supports the substrate in the chamber, a gas supply unit that supplies gas into the chamber, and a plasma generation unit that excites the gas in the chamber into a plasma state. The support unit includes a support plate on which the substrate is placed, a high-frequency power supply that supplies high-frequency power to the support plate, and a high-frequency transmission line through which the high-frequency power is supplied from the high-frequency power supply to the support plate. Characteristic impedance of the high-frequency transmission line is variable.