It is known that great Sanriku tsunamis sometimes hit the Sendai Plain in the north part of the Honshu Island in Japan. Among those great tsunamis, the field investigation had been rarely carried out about the tsunami of July 13th, 869 (11th year of the Jogan Era) because of the lack of the materials of old documents. We carried out the field investigation of the height of this tsunami in the Sendai Plain on the basis of the archaeological view and the sedimentologic examination. The former method is based on the archaeologist's judgement whether historic relics have the trace buried by the running water or not. We estimated the inundated area from the result of the survey for the 8 points in the Sendai Plain. In the latter method, we dipped the test pit and observed the soil layer of the side wall minutely to find the sand layer being supposed to be carried by the tsunami from the dune. Moreover to analyze the character of the sand layer, we made the physical examination, the chemical analysis and the measurement of the age by ¹⁴C method. From the results of these investigations, the height of the Sanriku Jogan 11 earthquake-tsunami (A. D. 869) is estimated as 2.5-3m at the point 3km from the coastline in the Sendai Plain. Finally we checked it with the social and geographic circumstances at that time, and we got the result that the estimated height is not inconsistent with the record of this tsunami.