An apparatus for treating a substrate comprises a process chamber having a processing space inside, a substrate support unit that supports the substrate in the processing space, a heating unit that heats the substrate supported on the substrate support unit, an exhaust unit that evacuates the processing space, and a guide member that guides a gas flow in the processing space. The guide member comprises a blocking plate that is located between an upper wall of the process chamber and the substrate support unit and spaced apart from an inner sidewall of the process chamber and that has a diameter smaller than an inner diameter of the process chamber. The exhaust unit is connected, at a position overlapping the blocking plate, to the upper wall of the process chamber when viewed from above.