The aim of this study is to clarify the sound absorption mechanism of porous asphalt pavement by comparing it with those of the glass wool and urethane foam etc., which are well-known porous sound absorbing materials. In order to elucidate the sound absorption mechanism, we measured the propagation constant and characteristic impedance of a sound wave traveling inside the material under the plane wave incident condition using an acoustic tube, and calculated the behavior of the sound waves in the material based on the measurement results. We concluded that the sound absorption of the porous asphalt pavement is caused by the following mechanism. Because the sound waves in the porous asphalt pavement material generally used in Japan exhibit less attenuation than those in glass wool or flexible urethane foam, the multi-reflected waves remain inside the material, and interfere with the wave reflected from the front surface of the material. In particular, in the frequency range below 1 kHz where the sound waves exhibit less attenuation inside the material, the sound absorption coefficient peaks at a frequency at which the antiphase condition is satisfied between the multi-reflective waves in the material and the sound wave reflected from the front surface of the material. Furthermore, the frequency range above 1 kHz is characterized in that since the attenuation gradually increases while the interference decreases, the sound absorption coefficient of the porous asphalt pavement is determined by its surface reflective wave.