A conventional automatic umbrella as shown in FIG. 1 includes: a plurality of ribs R pivotally secured to a lower runner L, a middle runner M and an upper runner U respectively, a central telescopic shaft including a lower slim tube S1 and an upper outer tube S2 having a diameter larger than that of the lower tube S1, and a grip G formed on a lower portion of the shaft having two spring catches C1, C2 for locking a folded umbrella and for shielding the tips T of the plurality of ribs R into the grip G when lowering the lower runner L.
Since the upper tube S2 is larger in diameter than the lower tube S1 and a central hole O inside the lower runner L is larger than an outside diameter of the upper tube S2, it will form a large aperture between the tube S1 and the runner L so that the lower runner L will not be stably engaged with either catch C1 or C2 and will be easily released from the catch C1, C2 to cause an unexpected opening of a folded umbrella.
The present inventor has found the drawbacks of a conventional automatic umbrella, and invented the present automatic umbrella having stable extending and folding structure.