Infrared spectra of protonated formic acid-water binary clusters, H+.(HCOOH)(n)H-. O (n = 1-5), are investigated by infrared photodissociation spectroscopy and A initio molecular orbital calculations. The asymmetric OH stretching vibration of water is observed in the infrared photodissociation spectra of the clusters with n = 1-3; it disappears in the spectra of the n = 4 and 5 clusters. On detailed comparison of the observed infrared spectra with calculated ones, the most stable geometric structures are obtained for the n = 1-5 clusters. These results suggest that the clusters switch the ion cores from HCOOH2+ for n = 1 - 3 to H3O+ for n = 4 and 5. The n = 5 cluster has a cyclic-type structure; the H3O+ ion core is fully surrounded and stabilized by five formic acid molecules. This characteristic nature produces a magic number of the n 5 cluster.