This paper describes the attenuation of a blast wave by a water column. After a water column had been generated by a previous under water explosion, a blast wave was caused by the detonation of a high explosive. The blast pressre passing through the water column were measured using piezoelectric gauges. The result showed that peak overpressure and scaled impulse were reduced by about 90 % and 80 %, respectively. It was concluded that the water column effectively attenuated the blast waves.