To provide a light quantity measuring device capable of measuring the quantity of light emitted from an optical system without increasing loss of the quantity of light, and to provide an illumination optical system, a projection optical system, an exposure apparatus and a device manufacturing method.

The light quantity measuring device 40 includes monitor units 41, 42 for monitoring scattered lights generated in lenses 29, 34 arranged on an optical path of an exposure light EL, and optical fibers 43, 44 for guiding the scattered light emitted from the side surfaces of the lenses 29, 34 to the monitor units 41, 42. From the monitor units 41, 42, electric signals equivalent to the light quantity of the scattered light received by photoelectric elements 41a, 42a are input into a controller 18, respectively. The controller 18 calculates the fluctuation of the light quantity of exposure lights EL emitted from the illumination optical system 13 and the projection optical system 16 in accordance with the electric signals input from each of the monitor units 41, 42.