Lotus-type porous magnesium whose long cylindrical pores were aligned in one direction was fabricated by unidirectional solidification of the melt dissolving hydrogen in a pressurized hydrogen atmosphere. The sound absorption coefficient of porous magnesium whose specimen face has many open pores was measured by standing-wave method in the range up to the frequency of sound of 4 kHz. The relationship between absorption coefficient and pore structure of porous magnesium was studied. The absorption coefficient increases with decrease of the pore size, while it increases with increase of the porosity. Moreover, the peak value with high absorption coefficient is shifted toward higher frequency of sound when the thickness of the porous magnesium specimen decreases.