In a photodetecting device 3, positional alignment marks 18A, 18B to serve as positional references of a photodetecting element 11 are formed at the front surface side of the photodetecting element 11. Moreover, a pin base 13 is provided with a threaded fitting pin 32 to be fitted with a cold plate 2, and the threaded fitting pin 32 is accurately positionally aligned with respect to the photodetecting element 11 via a positioning portion 33 positioned with respect to the positional alignment marks 18A, 18B exposed from a slit portion 23 and a cutaway portion 24 of a wiring board 12. Accordingly, in the photodetecting device 3, by only fitting the threaded fitting pin 32 with a recess portion 4 of the cold plate 2, the photodetecting element 11 is accurately positionally aligned with respect to the cold plate 2.