This paper provides a theory that pressure rising due to water hammer can be damped if smaller elastic air tubes are put in a pressure pipeline where water-hammer-associated waves propagate. In theoretical development, taken as a variable with small fluctuations responding to pressure fluctuations, the behavior of the tube is included in a term “the velocity of pressure waves”. According to an air ratio in the sectional area of the pipeline, a maximum value of water-hammer pressure is obtained from this theory. The results are summarized in a general-purpose chart. And, in the analyses and experiments conducted on the condition that a term “the behavior of cushion in a closed air cushion surge chamber” is considered to have large amplitudes, it is found pressure can be damped enough even with a smaller amount of air. For future study on the damping of pressure associated with the tube, this paper proposes the necessity to determine the range in which a term “pressure wave velocity” and a term “cushion behavior” function synergistically.