This invention relates to a closure cap for a fuel tank of an automotive vehicle, and, more particularly, it is concerned with such closure cap which can be effectively applied to the fuel tank of a motorcycle, for instance. 
In general, the closure cap device for a fuel tank in a motorcycle is of the so-called bayonet type, by which the closure cap is tight-fitted to the fuel pouring port of the fuel tank by rotating the same through a certain predetermined angle.  In this type of closure cap, however, unless the cap is rotated far enough, it tends to be readily loosened by its reverse rotation due to vibrations, etc.  caused by the vehicle during its running, and, in the worst case, the cap is dislodged and permits leakage of fuel from the pouring port of the fuel tank.  In spite of such disadvantage with such known type of cap closing system, it is usually difficult to recognize from outside whether the cap is in its perfectly tight-fitted condition to the pouring port, or not, and, moreover, it is difficult to notice that the cap is in its insufficient closure state (i.e., a state, in which the cap is insufficiently rotated).