A substrate processing apparatus includes a process chamber including upper and lower quartz walls, a substrate support disposed in the process chamber, radiant heaters respectively provided above and below the quartz walls of the chamber, and heat reflectors disposed outside the process chamber for reflecting heat towards the substrate support. Each of the heat reflectors has heating has a first thermally reflective section oriented to reflect the heat towards an outer peripheral region of the substrate support and a second thermally reflective section oriented to reflect the heat towards a central region of the substrate support. Each heat reflector also has a reflection angle adjusting mechanism by which an angle at which the second thermally reflective section reflects heat can be adjusted. The angle is adjusted depending on the temperature distribution across the substrate so that the substrate can be processed uniformly.