A cap capable of achieving enhanced safety by inhibiting the occurrence of sparks thereinside even if a lightning current has flowed through a fastener, a fastening structure using the same, and an aircraft including the fastening structure. The cap (27) is attached so as to cover a portion of a fastener (15) that couples an upper skin (3) of an aircraft and a stringer (11) located inside the upper skin. The covered portion projects through the stringer (11). An outer surface (33) of the cap that comes into contact with an interior space of the upper skin (3) is a curved surface, and the cap is made of a conductive material.