We estimated the scaling relation, i. e. M_O (seismic moment)-f_C (corner frequency) relation, of small to moderate-sized earthquakes east off northeastern Japan. We used spectral ratio method to accurately estimate f_C from observed spectra of earthquakes. We calculated spectral ratios of all the event pairs with spatial separations less than the hypocenter location errors, and estimated f_C values by fitting them with theoretical spectral ratios. Seismic moments were estimated from the JMA (Japan Meteorological Agency) magnitudes. We also estimated spatial variation of the scaling relation along the plate boundary of the subduction zone. Obtained scaling relation was compared with the scaling law derived by Nadeau and Johnson (1998) in which seismic coupling coefficient was assumed to be 100%. Obtained M_O-f_C samples are somewhat scattered and the distribution range corresponds to seismic coupling coefficients of 1 to 100%. The range corresponds to stress drops of 0.1 to 10MPa. Regional variations of stress drops are also observed. In particular, lower stress drops are estimated in and around the fault area of the 1896 Sanriku tsunami earthquake. Higher stress drops are obtained for the deeper portion (deep thrust zone) of the plate boundary. This tendency of higher stress drops for deeper events can be explained by the difference in physical properties (i. e., rigidity) which depend on depth. The M_O-f_C relation also varies along the arc. Areas with higher stress drops are distributed in and around the asperities of some large earthquakes. These events with higher stress drops might occur off the plate boundary and/or on the plate boundary in and around the asperities.