A smoke detector includes a substrate, an optical detection module, a base and a top cover. The substrate has a ring shape region surrounding a central detection region, and a first block structure of the central detection region is protruded from the substrate and higher than an upper surface of the ring shape region. The optical detection module is disposed inside the central detection region. The base is disposed on the substrate and around the optical detection module. The base has a second block structure. The top cover is connected to the base. A lateral wall of the top cover is partly overlapped with the second block structure to form a guiding channel. The optical detection module analyzes variation of scattering parameters resulted from gaseous matter entering the top cover through the guiding channel for determining concentration of the gaseous matter.