The photoelectric smoke sensor includes a casing member 1 having an internal space S formed by a labyrinth wall 1b and an opening 6 communicating with the internal space S on a setting-face side; and a printed board 8 placed on the setting-face side of the casing member 1 and having a hole 8a opposed to the opening 6 of the casing member 1. The light emitting element 2 and the light receiving element 4 are mounted on the printed board 8 in such a fashion that the optical axes of the light emitting element 2 and the light receiving element 4 cross each other on one plane that is substantially parallel to an opening plane of the opening 6. The photoelectric smoke sensor can be reduced in both size and cost with a simple construction.