To provide an air pressure sensor unit capable of easily replacing a battery in a short time and a tire air pressure monitoring system using the sensor unit.

In a valve cap 44, a battery 44b for supplying power to a sensor casing 42 and an IC chip 44c for recording sensor ID are molded. In the valve cap 44, a first terminal 44f in elastic contact with an annular contact 43d of a first conductive ring 43b and a third terminal 44h in elastic contact with an end face of a valve stem 41a are molded. The first terminal 44f is connected to a minus pole of the battery 44b, and the third terminal 44h is connected to a plus pole of the battery 44b. When the valve cap 44 is mounted on the tire valve 41, the first terminal 44f comes into elastic contact with the annular contact 43d of the first conductive ring 43b to supply power of stipulated voltage to each sensor 42a to 42c within the sensor casing 42, a CPU 42d and a transmitter 42e.