The present invention concerns a safety device for a radiator cap detachably mounted on a fluid supply opening for closing the radiator opening. The safety device comprises a lock means for preventing the opening of the radiator cap when the temperature of the coolant in the radiator is higher than a predetermined value. The lock means is provided between the radiator cap and an inner surface of the fluid supply opening. The lock means comprises a lock portion provided on an inner surface of the fluid supply opening and a lock member fixedly secured to the radiator cap. The lock member is formed of a shape memory alloy.