A gas cylinder residual gas volume indicator includes a carrier having a weight-receiving part and a pressure-applying part, a hydraulic cylinder having a cylinder body holding a hydraulic fluid and a cylinder piston stopped against the pressure-applying part of the carrier and movable with the pressure-applying part relative to the cylinder body to force the hydraulic fluid out of the cylinder body according to the weight received from a gas cylinder by the weight-receiving part, a hydraulic mechanical type meter having an index rotatable by a hydraulic fluid, and an oil pipe for guiding the hydraulic fluid out of the hydraulic cylinder into the meter to rotate the index.