Mudflow type failure of a gentle fill slope occurred at Dateshita, Tsukidate town during the 2003 Sanriku-Minami earthquake in Japan. The flowed fill material was pyroclastic sediment that was classified as a volcanic sandy soil with pumice. This study conducted laboratory shaking table tests to examine the change of the water retention nature that is attributable to vibration of this volcanic sand under an unsaturated condition. An electrical moisture sensor measured the change of the apparent volumetric water content of the soil mass with various water contents during and after shaking. The apparent volumetric water content was increased by shaking under the condition of a certain extent of water content. The extent of the water content corresponded to an in-situ one. Suction changes of the soil mass as a consequence of shaking were also tested from various initial suction states for both the drying and wetting processes. The suction change caused by shaking disturbance was observed. The suction change direction depends on the initial suction state. It is considered that the observed nature in two tests results, which is attributable to the peculiarity of the volcanic sandy soil with pumice, may be related to mudflow-type slope failure.