© 2018 The Japan Society of Applied Physics. A coherent anti-Stokes Raman scattering (CARS) spectroscopy system was designed to measure the transient behavior of the humidification of a Nafion membrane used in a polymer electrolyte fuel cell (PEFC). We observed a variation in the intensity of the O-H stretching vibration peak of water during dry and wet gas flow. From our measurements, we found that our system showed a high time resolution enough to measure humidification variation in a Nafion membrane in a PEFC. The high acquisition rate revealed the nonequilibrium process of the dehydration or hydration of the Nafion membrane through the temporal variation in the intensity of the O-H stretching vibration peak. The peak consisted of three peaks, suggesting that water molecules hydrated with different bonding energies. The transient behavior of the peaks was probably attributed to the motion of hydration molecules in the proton channel.