To provide a tire air pressure monitoring device that eliminates the need of mounting an initialization switch and the wiring to the receiver from the initialization switch, while solving the troublesomeness of users in the execution of the initialization compared with a necessary case of operation of the initialization switch.

The tire air pressure monitoring device has a receiver including a control part that obtains the air pressure in the tire based on the detection signal transmitted from the transmitter and operates the alarm when the air pressure of the requested tire falls below the warning threshold. The control part of the receiver is made to use the arbitrary set threshold that can be set to an arbitrary value as a warning threshold. When determined in the steps S12-S16 that the air pressure in the tire has changed from outside the predetermined range into the predetermined range when the vehicle stops, the arbitrary set threshold is calculated based on the air pressure of the tire after the change and the calculated arbitrary setting threshold is memorized.