PURPOSE: To prevent erroneous diagnosis and breakage of a fuel tank by constituting a safety valve out of a mechanical operation valve opened by predetermined set pressure in an evaporation purge system failure diagnosing apparatus for emitting evaporated fuel of an internal combustion engine to an intake system for processing.

CONSTITUTION: A fuel cap 4 of a fuel tank is composed of a threaded part body 14, outside body 15 and O-ring 16. The threaded part body 14 is composed of a hollow cylindrical part 14a having a threaded part to be fixed to the fuel tank, a crushing property diaphragm 14b for closing an opening of the hollow cylindrical part 14a and a disc part 14c disposed on the upper portion of the diaphragm 14b. The disc part 14c has an atmospheric port 14d and the upper surface of the diaphragm 14b is opened to the atmosphere, so that the diaphragm 14b is crushed when the internal pressure in the fuel tank becomes excessive negative pressure.