During the 2003 Sanriku-Minami earthquake in Japan, a flowslide was triggered on a slope of about 13 degrees. The displaced landslide mass deposited on a horizontal rice paddy after traveling approximately 130 m. Field investigation revealed that the landslide originated from a fill slope, where a gully was buried for cultivation some decades ago, and shallow ground water exists. To investigate the trigger and movement mechanism of this landslide, a series of seismic simulating tests was performed on a sample taken from the source area, by using a newly developed ring shear apparatus. The seismic loadings were synthesized using the seismic records of the same earthquake. The undrained and partially drained tests revealed that shear failure and post-failure landsliding may have resulted from the seismic loading, given the thickness of the saturated soil layer above the shear zone.