S³Fit fits spectrum and multi-band photometric Spectral Energy Distribution (SED) simultaneously for analyzing observational data of galaxies. It improves the moderate constraints on properties of continuum models in a pure spectral fitting due to the limited wavelength coverage. The code supports multiple models with multiple components, and can handle complex systems with a mixed contribution of Active Galactic Nucleus (AGN) and its host galaxy in both of continua and emission lines (e.g., narrow lines and broad outflow lines). The fitting strategy is optimized to enable an efficient solution of the best-fit results for several tens of parameters and model components. S³Fit is also extensible for adding functions and components by users such as new band filters, star formation history functions, emission lines, and also types of models.