The temperature dependence of the methane adsorption isotherm for two kinds of activated carbon fibers (ACFs) having different pore width above and below the critical temperature of bulk methane was measured. The average density rho (ad) of methane confined in micropores was estimated from the Langmuir pore filling amount of adsorption and the micropore volume. The temperature dependence of the rho (ad) indicated the possibility of an upward shift in the critical temperature of the confined methane. The upward shift of the critical temperature in smaller micropores was more remarkable.