To provide a highly sensitive photodetector hardly affected by the fluctuation of an optical axis, and capable of detecting light while holding a uniform light quantity distribution, and to provide a semiconductor exposure device using the photodetector.

A photodetector 10 is arranged on the optical axis of detected light DL. An optical path of the detected light DL is made therein not to be disturbed by a support frame 23 so as to make the whole detected light DL get incident into a self-standing conductive thin film 21. When the detected light DL of a short-wavelength light gets incident into the thin film 21, one part of light energy of the short-wavelength light is used for releasing photoelectrons, and a photocurrent equivalent to released charges flows between a detector main body 20 and a grounding. A light quantity attenuated in the thin film 21 and further a transmission light quantity are able to be measured by measuring the photocurrent by an ammeter 30.