A theft-proof bicycle comprising a frame structure consisting of three frame parts (1, 2, 3), which are connected so as to form a triangle, where the rod for the front wheel's supporting fork (4) is attached to the triangle's front corner (6), while the rear wheel's supporting fork structures are attached to the frame structure in the areas of the two other corners (7, 8) of the triangle. One of the frame parts (1) forming the triangle's front corner is designed with a holding sleeve for the rod for the front wheel's supporting fork (4). The sleeve is equipped with an extension and an aperture for passing through a locking pin. The second frame part in the area of this corner (6) is designed as a fork, one prong of which is passed on each side of the front wheel fork's rod. The supporting frame (1, 2, 3) is equipped in a corner (7) with a pivot joint in order to permit a relative movement of the supporting parts in the supporting fram's plane, between an operating or cycling position and a locked position.