Provided is a support unit for supporting a substrate, the support unit including: a heating member configured to transmit thermal energy to a supported substrate; a reflective plate disposed under the heating member and configured to reflect thermal energy generated by the heating member to the substrate; a cooling plate disposed under the reflective plate and formed with a cooling flow path in which a cooling fluid flows; and a gas supply line configured to supply gas to a space between the reflective plate and the cooling plate.