PURPOSE: To evade the danger due to continuous use of an emergency spare tire for a long period of time by a method wherein a detector sensing whether the spare tire is housed in its place or not is provided.

CONSTITUTION: A screw thread 4a equipped at the tip of a handle 4 is screwed into the main body 5 of the detector A. The detector A consists of the main body 5 made of insulating material equipped with two electric conductors 6 and 7 led therein, the inner end parts 6a and 7a of which are located on the upper surface of the interior of the main body 5. In addition a contactor 9, which is pushed up by means of a compression spring 8 and at the same time pushed down by means of the screw thread 4a equipped at the tip of the handle 4 against the compression spring 8, is located within the main body 5. One electric conductor 6 is grounded and the other electric conductor 8 is connected in series to an ignition switch 10, an electric power source 11 and an alarm such as an alarm lamp 12 or the like.