The author carried out the resistance test of prolate spheroids having the length-breadth ratio nearly 4, 6 and 8 at various immersion, and compared their residual resistance with the wave making resistance calculated theoretically. The values calculated by the first approximation theory are very lower than those from experiments, these difference may be explained in their character by the second approximation theory based on the perturbation method. But in our stage of the approximation, the calculated values are still far from correspondence with experimental values. Therefore he seeks and tries to ex plain these difference with the idea of the equivalent immersion, that is, these discrepancy is correspond to the condition shallower (or deeper) in the immersion. This correction in quantity is almost constant in high speed, and seems to give a fairly good estimation in other case.