A metal nanoparticle composite is provided, in which a matrix resin layer and metal nanoparticles are immobilized on the matrix resin layer. The metal nanoparticle composite has the following characteristics: a) the metal nanoparticles are obtained by heat-reducing metal ions or metal salts contained in the matrix resin layer or a precursor resin layer thereof; b) the metal nanoparticles exist within a region from the surface of the matrix resin layer to a depth of at least 50 nm; c) particle diameters of the metal nanoparticles are in the range of 1 nm to 100 nm with the mean particle diameter of greater than and equal to 3 nm; and d) a spacing between adjacent metal nanoparticles is greater than and equal to the particle diameter of a larger one of the adjacent metal nanoparticles.