The author investigated the vibration damping of the hydraulic system by the bladder type accumulator composed of rubber. First, observation concerning the deformation of the bladder was made. When the main pipe pressure is not so large compared with feed gas pressure of the Acc (abbreviation of accumulator), the rubber film moves axially like a solid piston. Secondly, the frequency response of the Acc itself was measured. When the inside profile of Acc neck is simple, measured natural frequency agrees well with calculated value. The damping of the Acc system is composed of oil flow and rubber bladder itself. Furthermore, the author investigated the 1st mode vibration damping method of the existing hydraulic elevator system of the cylinder-plunger type using the Acc and made clear the optimum design of Acc. Also, when the wave phenomenon is dominant, the author shows by the numerical calculation, the optimum insert position of Acc is the position of maximum pressure of the vibrational mode.