Photoacoustic measurement was applied to the monitoring of ink permeation into paper. As oil ink permeated into paper, PA signal decreased. The patterns of PA signal come out to be linear against the square root of elapsed time. This suggests that the PA signal follow the Lucas-Washburn equation. As W/O emulsion ink permeated into paper, the PA signal also decreased. And the W/O emulsion with larger viscosity showed slower PA signal decreases. But its PA signal change did not show as to be linear against the square root of elapsed time.