IR-dip spectra of trans-acetanilide-water 1:1 cluster, AA-(H2O)(1), have been measured for the S-0 and D-0 state in the gas phase. Two structural isomers, where a water molecule binds to the NH group or the CO group of AA, AA(NH)-(H2O)(1) and AA(CO)-(H2O)(1), are identified in the So state. One-color resonance-enhanced two-photon ionization, (1 + 1) RE2PI, of AA(NH)-(H2O)(1) via the S-1-S-0 origin generates [AA(NH)-(H2O)(+) in the D-0 state, however, photoionization of [AA(CO)-(H2O)(1)] does not produce [AA(CO)-(H2O)(1)](+), leading to [AA(NH)-(H2O)(1)](+). This observation explicitly indicates that the water molecule in [AA-(H2O)(1)](+) migrates from the CO group to the NH group in the Do state. The reorganization of the charge distribution from the neutral to the Do state of AA induces the repulsive force between the Water molecule and the CO group of AA(+), which is the trigger of the water migration in [AA-(H2O)(1)](+).