In the first and second report, comparisons between tank experiments and theoretical calculations were carried out mainly in following waves where encounter frequency is very low. At ω _e _??_0 transfer functions of vertical wave load from experiment showed a considerable difference compared with results of theoretical calculation methods. And this difference is understood that there was certain problem in calculation of diffraction force and radiation force, especially in following waves. In this paper, we made a reconsideration of accurate estimation on longitudinal ship motions and vertical wave loads taking account Mizoguchi's method for diffraction force and moment and using Maruo's three dimensional correction for only added-mass in radiation component. This calculation method for diffraction force and moment is a complementary approach in Newman's diffraction solution, considering no effect of the forward velocity on the hydrodynamic coefficients mentioned in our second report. By applying this method, experimental values of longitudinal ship motions and vertical wave loads can be explained satisfactorily not only in following sea, but in all the wave directions and all Froude numbers within the range for practical use.