On March 11 2011 a huge tsunami was caused by a magnitude 9.0 earthquake, which devastated the sea defenses in the Tohoku coastal region of Japan. We investigated one of the disaster areas, the Taro district, which had been very famous for having a 10m high seawall. This paper focuses on why the seawall in the Taro district collapsed during the tsunami. To this end the present work involves (1) the field disaster investigation, (2) the scale of the March 11 tsunami by performing its runup calculation, (3) the overflowing effects of the tsunami on the seawall by using the MPS (Moving Particle Semi-implicit) method, (4) the overflowing model test of a seawall and (5) the possible collapse mechanisms of the seawalls, in which they were mainly destroyed by the overflowing and the ensuing draw down wave of the tsunami.