PURPOSE: To prevent the destruction of the whole main wing structure, caused by excessive pressure generated in a fuel tank, without increasing the weight of an airframe and fuel consumption.

CONSTITUTION: A Teflon sheet 6 is disposed between a skin 3 and a beam 4 forming a fuel tank 5 in a main wing 1 integrally formed of compound material, and the skin 3 and the beam 4 are integrally formed and hardened. Accordingly, in the case of excessive pressure being generated in the fuel tank 5, the Teflon sheet 6 is destroyed, and the excessive pressure in the fuel tank 5 is lowered. The destruction of the whole main wing structure caused by excessive pressure generated to the fuel tank 5 can be thereby prevented by a simple device. and the manufacturing cost, the weight of an airframe and fuel consumption can be reduced.