A fail-safe shutoff device used in conjunction with a high-pressure control valve for compressed gas cylinders which will immediately check uncontrolled gas flow from the cylinder generally associated with a damaged or defective control valve. The shutoff device includes a valve means placed in series flow communication with the control valve and is pre-set to an open position and held open by balanced forces applied to opposite ends of the shutoff valve means. In this counterpoise condition, the valve means is subjected to the pressure of the compressed gas stored in the cylinder and is made responsive to a substantial imbalance of these forces, brought about by a damaged control valve to block escape of the compressed gas in the cylinder.