To provide an antitheft locking device to make a handle unable to rotate.

This antitheft locking device is provided with a means for inserting the cylinder A in the frame head part (called a head part hereafter) of a bicycle from above, and fixing the cylinder A to a head part, a means for inserting the cylinder B having the same diameter as that of the cylinder A in a handle fork part to stick it, winding a coil spring (called a linear coil) around the cylinder B, inserting it in the cylinder A fixed to the head part, and fixing the upper end of the linear coil to the cylinder A, a means for controlling its tip, which is formed by bending the lower end of the linear coil, and a handle stem linked to the handle fork is rotated, and a steering function is impeded by the enfolding friction of the linear coil on its inner surface, so as to make the handle unable to rotate.