Provided is a smoke detector having a structure in which, at a time of performing smoke detection, an S/N ratio of a signal of a light receiving element is improved, to thereby accurately detect occurrence of fire. At both end portions of an inside of an optical case (21) having a substantially cylindrical shape, a light emitting element (11) and a stray light portion (22) are arranged. In the stray light portion (22), a light trap (23) is provided. Further, in the vicinity of the light emitting element (11), a condenser lens (24) is provided to allow light L to incident on a curved surface of the light trap (23). At the time of smoke detection, light scattered by smoke particles in a smoke detection portion (25) is received by a light receiving element (12). However, the light incident on the light trap (23) is reflected on the curved surface a plurality of times to be attenuated in each reflection, so the light is not received by the light receiving element (12). Accordingly, the S/N ratio of an output signal S obtained from the light receiving element (12) is improved, thereby making it possible to accurately perform the smoke detection at an early stage.