An apparatus for exposing a wafer to light. The apparatus includes a light source unit having a light source for emitting light, a first cooling unit for cooling the light source unit, in which the first cooling unit has a gas flowpassage for a gas passing through the light source unit, a second cooling unit for cooling the gas, in which the second cooling unit has a first fluid flowpassage for a first fluid which is to be heat-exchanged with the gas in the gas flowpassage at a position downstream of the light source unit with respect to the flow of the gas, and a first temperature adjusting unit for adjusting the temperature of the first fluid. The first temperature adjusting unit has a second fluid flowpassage for a second fluid which is to be heat-exchanged with the first fluid in the first fluid flowpassage at a position upstream of the cooling unit with respect to the flow of the first fluid.