A polarization state detecting system includes a first dividing device for dividing incident light into two light beams having the same polarization state as that of the incident light, a detector for detecting one of the two light beams from the first dividing device, through a polarizer, and an acquisition device for acquiring information regarding the polarization state of the incident light on the basis of an output of the detector. The first dividing device includes a first element, a second element and a third element. The second element is disposed so that a p-polarization component reflected by the first element is reflected by the second element as an s-polarization component. The third element is disposed so that a p-polarization component transmitted through the first element is reflected by the third element as an s-polarization component, and one of the two light beams is light reflected by the first element and reflected by the second element, while the other of the two light beams is light transmitted through the first element and transmitted through the third element.