To provide a tire sensor that detects rotation of a tire, while suppressing increase of member count, and can change radio transmission frequency of a tire state according to rotation of the detected tire, and to provide a tire condition monitoring device using the tire sensor.

A sensor unit 3 as a tire sensor includes: a sensor unit controller 13 which measures a state of the tire which is attached to a wheel of a vehicle; and a transmitting circuit 14 which performs radio transmission of the data which shows a condition of the tire which the sensor unit controller 13 measured. The sensor unit 3 has a conductor in which either the sensor unit controller 13 or the transmitting circuit 14 is electrically insulated from the ground and the wheel, detects rotation of the wheel attached with the tire sensor based on an electrical condition of the conductor which varies according to change of distance of ground and semiconductor, and changes frequency of the radio transmission based on rotation of detected wheel.