In a method for manufacturing a spectral module 1, a photodetecting unit 10 constructed by bonding a photodetector 5 and a light transmitting plate 56 together is attached to a front face 2a of a substrate 2 by an optical resin agent 63. Here, a light transmitting hole 50 of the photodetector 5 is covered with a light transmitting plate 56, whereby the optical resin agent 63 is prevented from intruding into the light transmitting hole 50. When preparing the photodetecting unit 10, a semiconductor substrate 91 provided with a photodetecting section 5a and the light transmitting plate 56 are bonded together, and then the semiconductor substrate 91 is formed with the light transmitting hole 50, whereby matters which may cause refraction, scattering, and the like to occur can reliably be prevented from intruding into the light transmitting hole 50.