A system for directly measuring tire grip conditions is provided which can measure a skidding condition with no need of applying a vibration. A tire rotation speed sensor comprising a detection gear and a magnet sensor is mounted to a rotating shaft of a tire. An output of the tire rotation speed sensor is analyzed in time-frequency domain to detect vibrations caused by slips, thereby determining tire grip conditions, such as a slip ratio and a slip angle. By detecting fluctuations in rotation based on the cause-effect relation between the occurrence of tire slips and the occurrence of fluctuations in rotation, the tire slips can be directly detected. Therefore, the tire grip conditions can be confirmed with high accuracy.