Provided is a fire detector in which an element substrate can be easily removed from and mounted into an optical case. The fire detector includes: an optical case (21); an element substrate (31), on which a light-receiving element (12) is mounted, the element substrate (31) being provided in the optical case (21); a signal line (35) passing through an introduction portion of a peripheral wall (21a) of the optical case (21), the signal line (35) being connected to the element substrate (31); and an optical cover (30) for closing an opening of the optical case (21), in which the signal line (35) is inserted into a slit (37) which is open on an upper end surface (21f) of the introduction portion of the peripheral wall (21a).