A spectrometer capable of eliminating side-tail effects includes a body and an input section, a diffraction grating, an image sensor unit and a wave-guiding device, which are mounted in the body. The input section receives a first optical signal and outputs a second optical signal travelling along a first light path. The diffraction grating receives the second optical signal and separates the second optical signal into a plurality of spectrum components, including a specific spectrum component travelling along a second light path. The image sensor unit receives the specific spectrum component. The wave-guiding device includes first and second reflective surfaces opposite to each other and limits the first light path and the second light path between them to guide the second optical signal and the specific spectrum component. The first and second reflective surfaces are separated from a light receiving surface of the image sensor unit by a predetermined gap.