Recently, a conceptual design study of the displacement-type high speed ship has been presented. For a new type ship, a serious problem may very possibly occur in relation to the ship hull strength, especially, on the longitudinal one which is the most fundamental hull strength. The elastic backbone model tests and the numerical analyses have been carried out in order to investigate the longitudinal strength of the ship, especially, in the high speed condition. Based on the above mentioned investigation, both the problem at high speed and the safety in the rough sea of the longitudinal strength of this ship have been discussed.