A micro tsunami from an interplate earthquake (Mw 6.1) was observed in 1998 on ocean bottom tsunami meters (OBTMs) deployed east off the northeastern Japan. The offshore tsunami data without complex distortions due to the coastal topography enable us to estimate reliable tsunami source parameters. The observed amplitude was about 1.5 cm at epicentral distance less than 100 km. We numerically computed the tsunami waveform by solving the linear Boussinesq equations. The observed tsunami waveforms are well explained by synthetic waveforms assuming the fault width of 10 - 15 km. The depth of the fault is estimated as 5 - 10 km below the seafloor, which is in good agreement with the location of the plate boundary defined by previous seismic studies.