[Technical Problem] To provide a differential evacuation system capable of easily maintaining, at a low cost, a large differential pressure between a light generation chamber and an illumination optical chamber in which optical processing, e.g. exposure, is performed by using extreme ultraviolet (EUV) light generated in the light generation chamber, and yet capable of sufficiently ensuring a desired optical path.[Solution to Problem] The differential evacuation system has a light generation chamber 10 that generates EUV light, an illumination optical chamber 100 in which optical processing is performed by using the EUV light generated in the light generation chamber 10, and a chamber connecting passage 150 that connects together the light generation chamber 10 and the illumination optical chamber 100 to guide the EUV light generated in the light generation chamber 10 into the illumination optical chamber 100. The chamber connecting passage 150 has a flow path constricting portion 151 and is increased in inner diameter in a conical tube shape at portions thereof that are at opposite sides, respectively, of the flow path constricting portion 151. An enlarged-diameter part 160 is provided at a position of the chamber connecting passage 150 that is closer to the light generation chamber 10, which is the higher in pressure of the two chambers 10 and 100, than the flow path constricting portion 151, and vacuum pumps 170 are attached to the enlarged-diameter part 160.