Disclosed herein is a built-in RFID tire tag. The built-in RFID tire tag includes an antenna and a circuit unit. The antenna is mounted in a tire, and is configured to be operated by resonating with radio waves, which are transmitted from an RFID reader and to have a repetitive corrugated shape. The circuit unit is configured to be operated by being supplied with power using the radio waves. The antenna includes a pair of tag radiators. Each of the tag radiators is divided into a first region and a second region. The corrugation density of the second regions, which are close to the circuit unit, is greater than that of the first regions, which are not close to the circuit unit. The first regions, which depend on rotational motion of the tire, adapt flexibly to bending and torsion of the tire by the difference between the corrugation densities.