To realize collective measurement of ultrawide-band optical signals which have exceeded an electrical band limit. This photodetection device (100) comprises: a time-delay adjustment means (10) into which is input an optical signal that is a detection object; an optical frequency comb generator (20) that generates an optical frequency comb; a photomixer (30) that photomixes optical signal series which are sequentially output from the time delay adjustment means (10) and which were subjected to time adjustment, and the optical frequency comb signal which is output from the optical frequency comb generator (20); and a photodetector (40) that detects the mixed signal output from the photomixer (30). The time delay adjustment means (10) performs time adjustment on the optical signal series so that each of the time adjusted optical signal series is photomixed with the optical frequency comb at different timings in the photomixer (30).