A safety cap device for controlling a pressure in a radiator includes a valve seat provided with a valve receiving space in which a first communicating hole is formed. A negative pressure valve is provided movably in the first communicating hole. A seal is disposed on a receiving surface of the valve seat and disposed movably with the negative pressure valve. The seal has a second communicating hole formed at a central portion thereof. A positive pressure valve is disposed on the seal and has a third communicating hole formed at a central portion thereof. A cap covers an upper portion of the valve seat. A spring is interposed between the positive pressure valve and the cap. A fluid receiving part is in the valve seat to prevent cavitation caused when an internal fluid expands.