PURPOSE: To detect the pressureless condition of a tire during running by passing a detecting element of a measuring device through a rim bottom to be projected into a tire internal space, and operating the detecting element in response to the approach of the tire inner surface to the rim bottom to output a signal.

CONSTITUTION: A push rod 8 is disposed in such a manner as to pierce through the rim bottom 3 and project into the tire internal space 9, and the other end of the push rod 8 is connected to a sensor 14 through a transmission mechanism 11 and an operating element 13. In this arrangement, the tire internal face is brought into contact with the push rod 8, whereby the push rod 8 is moved to the rim bottom 3 in the radial direction. This movement activates the transmitting mechanism 11 and an operating element 13, and the activation is detected by the sensor 14 to give a warning and make a display. By this arrangement, the pressureless condition of the tire can be measured in addition to a lowering of tire pressure during running.