Infrared spectra of jet-cooled 5-methoxysalicylic acid (5-MeOSA) in the 3 μm region were measured for both the electronic ground (S₀) and first excited (S₁) states. Effects of the methoxy-substitution were investigated in relation to the excited state intramolecular proton transfer (ESIPT) of salicylic acid. Though the electronic absorption - emission spectra exhibit the mirror-image relation, which has been regarded as evidence for the suppression of ESIPT, the IR spectra showed an extremely large elongation of the phenolic O-H bond upon photoexcitation. The results represent that the significant enhancement of the intramolecular hydrogen bond does occur in S₁ of 5-MeOSA.