We have prepared colloidal noble metals (Ag, Au, Pt) by photoreduction of metal ions in solutions and examined their optical properties on the basis of Mie theory. Tractable expressions for the extinction coefficient, K _ext, scattering coefficient, K _sca, absorption coefficient, K _abs, have been introduced by expanding the denominator and numerator of Mie coefficients respectively in power series of a size parameter and assembling them. General features of calculated extinction spectra appear to be in nice accordance with the observed ones ; extinction maxima located at λ≅410 nm (Ag) and 540 nm (Au), and even optical density almost coincided each other in colloidal gold and platinum. It has been confirmed that our method is also useful to estimate the optical properties of colloidal semiconductor (AgCl) and insulator (SiO₂) in water. The present method seems to be superior to the conventional van de Hulst's equation because of its applicability to the colloidal system whose size parameter is larger than 0.6. Preparation, of complex colloids such as Ag/Au, Au/Pt, Ag/Pt and Metal/SiO₂ was attempted and their optical properties were examined by introducing an effective medium theory to estimate the optical constants of composite materials.