It should be enabled to transmit tire-pressure information also on a spare wheel to a tire-pressure notification device, while suppressing battery consumption. A transmission controller of a sensor unit makes a transmitter periodically transmit wheel information, when an acceleration Gx detected by an acceleration sensor is larger than a running-judgment threshold value 5G. The transmission controller accumulates a point P set up according to the acceleration Gx in a predetermined period when the acceleration Gx is not more than the running-judgment threshold value 5G, and makes the transmitter transmit the wheel information when an accumulated value of the point P accumulated in the predetermined period is larger than a running-track-record-judgment threshold value.