A safety cap (10) which is screwed onto an external thread (24) of standard tire valve (22) consists of a cylindrical body (11), a cup-shaped barrel (26) which is threaded onto an external thread (17) of the cylindrical body (11), and a control valve which consists of a plunger (42), a valve seat (50), a spring (46), and a guide pin (40). This valve is located in a space (B) formed between the inner face of a barrel bottom (30) and the facing end of the cylindrical body (11). The control valve is constantly adjusted, during the assembling of the safety cap at a factory, to a maximum pressure allowable for a predetermined tire. In case the pressure in the tire exceeds the maximum allowable value, e.g., because of an increased load or high temperature air will push the plunger (42) away from the tire valve and will be released into the atmosphere through a check valve (62). Thus the pressure in the tire will be automatically adjusted to a level not exceeding the maximum allowable one.