A spectrometer 1A includes a package 2 having a stem 4 and a cap 5, an optical unit 10A disposed on the stem 4, and a lead pin 3 for securing the optical unit 10A to the stem 4. The optical unit 10A includes a dispersive part 21 for dispersing and reflecting light entering from a light entrance part 6 of the cap 5, a light detection element 30 having a light detection part 31 for detecting the light dispersed and reflected by the dispersive part 21, a support 40 for supporting the light detection element 30 such that a space is formed between the dispersive part 21 and the light detection element 30, and a projection 11 protruding from the support 40, the lead pin 3 being secured to the projection 11. The optical unit 10A is movable with respect to the stem 4 in a contact part of the optical unit 10A and the stem 4.