The reaction dynamics of excited-state intramolecular proton transfer (ESIPT) of 2,2'-dihydroxyazobenzene (2,2'-DHAB) was investigated by means of white-light supercontinuum femtosecond transient absorption spectroscopy. A coherent in-phase oscillation was observed in the entire wavelength range where stimulated emission of the photoproduct is dominant. This result indicates that the transition strength of the product state is dynamically modulated by a nuclear wavepacket motion (non-Condon effect). The observed vibration was assigned to the mode which modulates the distance between oxygen and hydrogen atoms. By integrating the result of time-dependent density functional theory calculation, the origin of the non-Condon effect was attributed to a dynamical change of configuration interaction between enol and keto characters along the vibrational coordinate, indicating that this vibration is strongly related to the reaction coordinate of ESIPT.