A safety device, in particular for a cylinder for holding gaseous fluids under pressure for a self- propulsion gas system, comprising: - a device body, in which a passage is defined for a fluid, said passage extending from an entry aperture, suitable for being placed in fluidic communication with said cylinder, to an exit aperture; - an obturator airtightly engaging said passage and acting in a continuous manner depending on the fluid pressure, said obturator being movable from a first position, in which it indicates the presence of fluid at a pressure below a first predefined pressure threshold value inside said cylinder, and a second position, in which it indicates the presence of fluid at a pressure above said first predefined pressure threshold value inside said cylinder; - at least one temperature and/or pressure-sensitive relief device (12, 12') positioned in said obturator in such a way that, upon exceeding a predefined temperature threshold or upon exceeding a second predefined pressure threshold, said relief device (12, 12') opens the passage for the fluid and permits the flow of fluid from said entry aperture towards said exit aperture; said safety device being characterized in that the relief device (12, 12') opens the passage for the fluid independently of the position of the movable obturator; in that said exit aperture is open towards the outside environment; in that the obturator has a head which engages said exit aperture in such a way that when the obturator is in the rearward position, said head is hidden from sight, whereas when the obturator is in the advanced position, said head is visible from the outside; and in that the safety device further comprises a shut-off valve of the flow suitable for intercepting the flow of fluid coming from said cylinder upstream of the obturator. - a device body, in which a passage is defined for a fluid, said passage extending from an entry aperture, suitable for being placed in fluidic communication with said cylinder, to an exit aperture; - an obturator airtightly engaging said passage and acting in a continuous manner depending on the fluid pressure, said obturator being movable from a first position, in which it indicates the presence of fluid at a pressure below a first predefined pressure threshold value inside said cylinder, and a second position, in which it indicates the presence of fluid at a pressure above said first predefined pressure threshold value inside said cylinder; - at least one temperature and/or pressure-sensitive relief device (12, 12') positioned in said obturator in such a way that, upon exceeding a predefined temperature threshold or upon exceeding a second predefined pressure threshold, said relief device (12, 12') opens the passage for the fluid and permits the flow of fluid from said entry aperture towards said exit aperture; said safety device being characterized in that the relief device (12, 12') opens the passage for the fluid independently of the position of the movable obturator; in that said exit aperture is open towards the outside environment; in that the obturator has a head which engages said exit aperture in such a way that when the obturator is in the rearward position, said head is hidden from sight, whereas when the obturator is in the advanced position, said head is visible from the outside; and in that the safety device further comprises a shut-off valve of the flow suitable for intercepting the flow of fluid coming from said cylinder upstream of the obturator.