This invention provides a tire warning device which detects tensile forces of at least one of reinforcing cords of a carcass ply or belt ply of a pneumatic tire and provides a warning on the basis of the tensile forces detected. The tire warning device includes a strain sensor to be attached to the at least one reinforcing cord for sensing a strain of the at least one reinforcing cord, transmitting means for transmitting a signal sensed by the strain sensor, receiving means for receiving a sensed signal from the transmitting means, processing means that calculates the tensile forces of the at least one reinforcing cord on the basis of the sensed signal input from the receiving means and determines whether the warning is provided or not on the basis of the calculated tensile forces, and warning means for providing the warning based on the determination of the processing means.