A tire changing machine having a self-centering platform with a vertical axis is provided with a safety device for releasably locking on the platform a wheel with a tire. The device is particularly useful during tire-inflating operations to prevent an operator from suffering injury due to an explosion of the tire. The device comprises a stem which is vertically slidable with respect to a horizontal arm, the stem inferiorly bearing a plate interacting contactingly with the wheel placed on the platform. A retaining organ exhibits a hole in which a vertical rod which is solidly constrained to the arm is inserted with a predetermined degree of play. The retaining organ is hinged to the stem in such a way as to be able to assume at least one locked position in which it prevents the stem from rising.