The water hammerless opening device comprises an actuator operating type valve installed on the fluid passage, an electro-pneumatic conversion device to supply the 2-step actuator operating pressure Pa to the afore-mentioned actuator operating type valve, a vibration sensor removably fixed to the pipe passage on the upstream side of the actuator operating type valve, and a tuning box to which the vibration detecting signal Pr detected by the vibration sensor is inputted, through which the control signal Sc to control the step operating pressure Ps′ of the afore-mentioned 2-step actuator operating pressure Pa is outputted to the electro-pneumatic conversion device, and with which the 2-step actuator operating pressure Pa, of the step operating pressure Ps′ which makes the vibration detecting signal Pr nearly zero, is outputted from the electro-pneumatic conversion by adjusting the control signal Sc.