Low pressure oxygen or other breathable gas is supplied from a tank or other source to a patient or other person through a flexible plastic tube, and a safety valve system is connected between the source and the tube. The valve system includes a valve member having a tubular outlet portion and supported for axial movement within a valve body having an inlet connected to the gas source. Fluid passages are formed within the valve member and valve body, and the valve member shifts axially to an open position in response to connecting the flexible tube to the tubular outlet portion of the valve member. The valve member shifts to a closed position in response to removing the tube from the tubular outlet portion, and the supply gas holds the valve member in the closed position.