Mechanism of SO2 micropore filling in the slit-shaped micropores of activated carbon fiber (ACF) was studied by SO2 adsorption calorimetry and molecular potential calculation. The pore width of ACF was 0.75 nm from analysis of N-2 adsorption at 77 K. The characteristic change of the differential adsorption energy for SO2 with increase of the fractional filling showed the presence of specific interaction of the SO2 dipole with the surface functional group, enhanced SO2-micrographitic wall interactions with the induced dipolar interactions, and permanent dipole-dipole interactions. The contribution of the dipole-induced dipole interactions calculated by the image potential approximation was 25% of the whole interaction potential of SO2 with micrographitic walls, which agrees with the observed adsorption energy. The strong SO2-micropore wall interaction indicated a ferroelectric-type orientation of SO2 molecules.