Electronic spectra of benzene-(water)_n cluster cations ([Bz (H₂O)_n]⁺, n = 1–4) in the visible and near ultraviolet regions were measured. The spectral feature of [Bz-(H₂O)_1–3]⁺ is similar to that of bare Bz⁺, showing that the electronic structure of the benzene cation is preserved in the cluster cations up to n = 3. At n = 4, however, a remarkable decrease of the absorption intensity takes place, indicating an essential change of the electronic structure of the benzene moiety. This result is consistent with the size dependent intracluster proton transfer reaction which has been suggested by the previous infrared spectroscopic study.