Bouguer gravity anomaly observations revealed the basement relief under the western flank of Daisen Volcano. A total of 250 observation points including 150 new points in the studied area with 9x10 km in size describe a N-S basement ridge in this area after the band-pass filtering of 1-20 km in wavelength. In addition, three point array observations of microtremor showed that the Daisen volcaniclastic sediments covering the basement rocks consist of 2 or 3 layers in this area. Single point microtremor observations also revealed predominant periods, which nearly represent the Daisen volcaniclastic sediments. The thickness changes of these sediments are in good agreement with the basement relief estimated from the gravity anomaly.