Co-seismic fault slip distributions of the 2011 Tohoku-Oki earthquake and interseismic couplings prior to the earthquake were estimated from surface 3-dimensional displacements measured by the GPS Earth Observation Network (GEONET). The deduced co-seismic slip was up to 30 m near the epicenter, and greater than 5 m in a 400 x 200-km region extending along the Japan Trench. The moment magnitude of the slip was estimated to be M(w)9.0. The area of co-seismic slip greater than 10 m overlaps with four previously known M7-class asperities. We researched inter-plate coupling on the fault for 15 years prior to the earthquake using daily GPS data. We determined that the plate boundary was locked during at least the last 15 years in a limited area of a few 10 s of km, approximately coinciding with the maximum co-seismic slip area, suggesting that this small area had been persistently locked and that the rupture of this last patch played an important role to cause the mega-thrust earthquake. The low seismic-coupling which is ratio of seismic slip to slip deficit accumulation of M7-class earthquakes in this area, which were reported by a study using seismic wave analysis over the past 80 years, can be explained by the influence of this locked area which may have dragged the hanging wall and suppressed the effective slip deficit in surrounding area.