OH and NH stretching vibrations of 2-naphthol-(NH₃)_n (n=1-3) hydrogen (H)-bonded clusters in the electronic ground state have been observed by IR-UV double-resonance spectroscopy. Cluster size dependence of the OH stretching vibration was investigated by comparing the observed spectra with the simulated ones obtained by ab initio molecular orbital calculations at the HF/6-31G and HF/6-31G(d,p) level. For the n=1 cluster, having the trans-linear structure, the band of the H-bonded OH stretching vibration exhibits a remarkably large low-frequency shift as large as 400 cm^-1 from that of bare 2-naphthol. For the n=2 and 3 clusters, having a ring-form of hydrogen bonds, the band of the OH stretching vibration showed further large red-shifts (∼ 700 cm^-1). In addition, the largely red-shifted bands become extremely broad. The results represent the extensive overlap of many bands which appeared by anharmonic coupling with OH stretching vibration, IVR process.