Colloidal gold (Au) nanoparticles were prepared and successfully loaded on titanium(IV) oxide (TiO2) without change in the original particle size using a method of colloid photo-deposition operated in the presence of a hole scavenger (CPH). The prepared Au nanoparticles supported on TiO2 showed strong photoabsorption at around 550 nm due to surface plasmon resonance (SPR) of Au and exhibited a photocatalytic activity in mineralization of formic acid in aqueous suspensions under irradiation of visible light (>ca. 520 nm). A linear correlation between photocatalytic activity and the amount of Au loaded, that is, the number of Au nanoparticles, was observed, indicating that the activity of Au/TiO2 plasmonic photocatalysts can be controlled simply by the amount of Au loading using the CPH method and that the external surface area of Au nanoparticles is a decisive factor in mineralization of formic acid under visible light irradiation. Very high reaction rates were obtained in samples with 5 wt % Au or more, although the rate tended to be saturated. The CPH method can be widely applied for loading of Au nanoparticles on various TiO2 supports without change in the original size independent of the TiO2 phase. The rate of CO2 formation also increased linearly with increase in the external surface area of Au. Interestingly, the TiO2 supports showed different slopes of the plots. The slope is important for selection of TiO2 as a material supporting colloidal Au nanoparticles.