In this study, we have photochemically fabricated a gold cluster (Au-n), composed of several tens of atoms, in a polymer matrix and concurrently investigated the photoreactivity of newly formed clusters at the single-cluster level. Our attempt enabled the in situ observation of individual ligand-free Au-n clusters with a resolution of the number of atoms. In addition, the procedure revealed the photoinduced reaction of excited Au-n clusters with O-2 for the first time.