To provide a worn tire warning device capable of warning the worn tire automatically.

A worn tire information process unit 71 obtains running distance for a certain interval from the position of a vehicle detected by the GPS and based on the running distance and the number of rotations of the wheel detected by a rotation number sensor 3, the outer circumference CN of the worn tire and outer diameter DN of the worn tire are operated; a before worn information process unit 72 operates the outer circumference CI and the diameter of the before worn tire DI; the worn amount of the tire process unit 73 compares the outer circumference CN and outer diameter DN of after worn tire and the outer circumference CI and outer diameter DI of before worn tire inputted from the before worn tire information process unit 72 and calculates the differential worn amount ΔC and ΔD; a dangerous worn state determination part 75 generates an alarm signal H, warns to the driver by activating the voice output part 4 and the display 6, when the worn amounts ΔC and ΔD exceed the threshold value ΔCth and ΔDth inputted from a dangerous worn amount determination part 74.