The dynamics on the vacuum-ultraviolet one-photon ionization of a formamide－water cluster is investigated by a combination of theoretical reaction-path search and infrared spectroscopic methods. A keto－enol tautomerization of the formamide moiety occurs after photoionization by a catalytic action of the water molecule accompanied with its long-distance migration; the water molecule in the cluster migrates almost one turn around the formamide moiety. During the migration, the water molecule abstracts the proton of CH in the formamide moiety and carries it to the O atom side in the carbonyl group through a "catch and release"-type catalytic action.