Abstract: A light scattering type smoke detector minimized in size and protected from erroneous operation and from internal accumulation of water drops is provided. The optical comprises light emitting and receiving elements and light shielding means which are compactly mounted on a base plate at different heights so as normally to prevent light from the emitting element to reach the receiving element. A power source part for the light emitting element disposed at one side of the optical system and an amplifying part for the light receiving element disposed at the other side of the system are housed in an upper lid having drain means, and a lower lid having many small holes for smoke lead-in is attached to the upper lid.