When a ship is going on waves its resistance increases compared with the same speed on smooth water, this fact has been studied experimentally by Kent, Kempf, etc.The author tried to analyse this phenomena theoretically, applying Michell's theory of wavemaking resistance on waving surface of water at every instant of oscillations of a ship, and investigated the following items, i.e., (1) the effects of wave height, wave length, and waves apparent period on the increase of resistance, (2) the effects of ship's speed, length, draught, water-line coefficient, block coefficient on the increase of resistance, (3) resistance amplitude varying periodically among waves, (4) the critical values of Froude number, (5) the effects of bow shape or Maier form on the increase- of resistance on waves.These results are proved fully by the experimental facts. The reason of increasing resistance is exclusively due to the heaving motion of the ship, or in other words, the periodical change of displacement by the oscillation of the ship on waves, and so, the pitching motion does not contributes too much.The resistance becomes very large when heaving or pitching falls in resonance, or when wave length is equal to the ship's length, and is proportional to the square of the wave height, but decreases when ship's speed becomes high, and ship's form becomes fine.