This report deals with the problem of longitudinal hull vibration of ships. Since the position of neutral axis for vertical bending does not coincide with the position of centre of mass in the usual ships, the longitudinal vibration couples with vertical vibration. In the first part of the report the equation of motion for coupled vertical-longitudinal vibration is derived and the numerical solution technique is applied to the equation. By means of this solution the qualitative nature of coupled vertical-longitudinal vibration is discussed. In the next part the effect of such local vibration as at deck house and shafting system upon the longitudinal vibration is examined. In the last part the calculation of natural frequencies of coupled vertical-longitudinal vibration is carried out for three actual ships and the results are compared with the experiment. It is concluded that, although the coupling effect well explains the phenomenon on board actual ships, the shift of natural frequencies due to coupling is not so remarkable for actual ships. The calculated frequencies agree with experimental ones by about 10% of errors.