In this study, the effectiveness of an offshore breakwater for the 2011 off the Pacific Coast of Tohoku Earthquake Tsunami was examined by quasi three dimensional (quasi-3D) and three dimensional (3D) numerical models. First, both 3D numerical models were applied to the behavior of tsunami inundation inside of Kamaishi bay where a baymouth breakwater installed against to assumed tsunami. The numerical results well agreed with the post-event tsunami survey on the land. The numerical simulations were performed without offshore breakwater. It is found that the offshore breakwater performed significant reduction of the tsunami height on the land. Furthermore, the velocity of tsunami inundation was discussed for understanding physical characteristics of tsunami around Kamaishi bay.