Visible-light photocatalysts are promising materials for car cabins, as they exhibit multiple functions under indoor light conditions, including antimicrobial and antiviral activities. Owing to the increasing demands for fast-acting sanitary materials, we studied the antimicrobial activities of nitrogen-doped titanium oxide loaded with Cu (Cu/N-TiO₂), a visible-light photocatalyst, in short time reactions with Escherichia coli. The survival rate of E. coli on a Cu/N-TiO₂-coated surface rapidly declined to 0% after 60 min, as assessed using a newly developed assay system and three types of resins. The morphological changes observed in E. coli resulted from the production of radicals after light exposure.