An apparatus for treating a substrate includes a process chamber having a treatment space defined therein, a support unit for supporting the substrate in the treatment space, a liquid supply unit for supplying treating liquid to the substrate supported on the support unit, and a heating unit disposed in the support unit for heating the substrate supported on the support unit, wherein the heating unit includes a plurality of lamps to heat the substrate, and a window disposed above the lamps to transmit light emitted from the lamps, wherein the window includes a base in a form of a plate, and light adjustment means formed on the base to spread or converge light emitted from the lamps.