To provide a tire air pressure monitoring device capable of constantly grasping the air pressure of a tire of a vehicle, and determining whether or not any trouble occurs in the tire by a user.

Every time when a monitoring ECU 54 acquires the identification data and the air pressure data from a signal separation circuit 52, it forms the data set together with the time data and stores it in a memory 53. When the air pressure display signal is received from a car navigation device 6, the monitoring ECU 54 prepares a graph to indicate the transition of the air pressure of tires A-D based on each data set stored in the memory 53, and displays it on a display 61 of the car navigation device 6. A user can determine whether or not the air pressure of the tires is rapidly dropped in a short time, in other words, whether or not any trouble occurs in the tires. The air pressure of the tires at the present time can be checked from the displayed graph.