Abstract: An apparatus for processing a substrate according to the present invention comprises a lamp unit heating the substrate placed in the chamber at a position facing the substrate. A transmission window constituting the top wall of the chamber and transmitting light emitted from the lamp unit is provided between the chamber and the lamp unit. A window assembly having a wall constituted by the transmission window is provided at the lamp unit side of the transmission window. An evacuation unit is connected to the window assembly. A pressure control unit controls the evacuation unit to maintain the internal pressure of the window assembly at a specific pressure. In this way, multiple substrates are subject to a significantly uniform substrate processing when they are processed in succession.