To provide a tire monitoring system capable of informing occupants that the state of each tire is unbalanced.

The measured pneumatic pressure Pn of each tire is calculated, and the average value Pave of the measured pneumatic pressure Pn is calculated. The average value Pave is compared with the measured pneumatic pressure Pn, and if the differential pressure of the warning differential pressure a is present, it is informed that he pneumatic pressure is unbalanced. An occupant of a vehicle recognizes an unbalanced state of the tire pneumatic pressure in an early stage which causes deviation of a worn state of each tire and degradation of the steerability, and countermeasures such as adjustment of the pneumatic tire of each tire can be taken.