The method of statistical estimation for irregular amplitude of general oscillation of a ship on waves is introduced by St. Denis and Pierson. The author applied this idea actually with Neumann's spectrum distribution to ship's rolling velocity and acceleration, considering incompletely arizen sea surface by Sverdrup and Munk's theory. The analysed results are compared with voyage data of the training ship Hokuto-maru, explain macroscopically the nature of rolling motion. Number of swings in which one complete resonance amplitude, velocity, and acceleration may be statistically included is : φ ( N ) =1/ F , G , H . where F, G, and H are ratio of cumulative amplitude, velocity, and acceleration to those of resonanced value, and φ is the maximum value derived from Longuet-Higgins's theory. According to analytical results, F < G < H so that irregularity decrease for higher derivatives of rolling motion. On rough seas and swells, though perfect 'resonance amplitudes might occur rarely, but perfect resonance accelerations occur frequently.