The photoreaction of 2-amino-5-methylpyridine was investigated by matrix-isolation infrared spectroscopy and DFT calculation. Photoinduced reversible amino (N=C-NH2)-imino (NH-C=NH) tautomerism was found between 2-amino-5-methylpyridine and 5-methyl-2(H-1)-pyridinimine; the amino tautomer changes to the imino tautomer by UV irradiation ( 340 > lambda >= 300 nm) and the reverse change occurs by longer-wavelength light irradiation ( 420 > lambda >= 340 nm). The results of the CASSCF calculation revealed that the amino-imino tautomerism proceeds in vibrational relaxation process from electronic excited state to the ground state. The IR spectra of 2-amino-5-methylpyridine in the T-1 state and 5-methyl-2-pyridinamino radical were also obtained by UV irradiation (lambda >= 300 nm).