Silver nanoparticle-silver halides (Ag-AgX, X = Cl, Br, I) have been attracted much attention as a new type of visible-light photocatalyst for solar-to-chemical transformations. Improving the activity of the “plasmonic photocatalysts” requires the compatibility of local electric field enhancement (LEFE) and effective absorption of the sunlight. Ag nanoparticle (NP) possesses strong LEFE effect, while the localized surface plasmon resonance (LSPR) peak located near the blue edge of the visible region. On the other hand, the absorption of Au NP well matches with the solar spectrum, but the LEFE is much smaller than that of Ag NP. Aux-Ag1-x alloy NP varies the optical property between those of Ag and Au NPs depending on the composition x, and thus, Aux-Ag1-x alloy NP-incorporated AgBr (Aux-Ag1-x@AgBr) can be a promising plasmonic photocatalyst. At the first step, a successive ionic layer adsorption and reaction (SILAR) method forms gold ion-doped AgBr NPs on mesoporous TiO2 film. At the second step, UV-light irradiation to the sample in methanol yields Aux-Ag1-x alloy particles having diameter of ~5 nm in the interior of AgBr with crystallite size of ~50 nm. The LSPR peak position can be tuned in the range between 500 nm to 600 nm by the alloy composition. Aux-Ag1-x@AgBr/mp-TiO2 exhibits a high level of photocatalytic activity under visible-light illumination.