Abstract: An operation condition detecting system for an automobile vehicle includes a fifth wheel rotatably connected to a shaft protruding downward from the bottom center of an automobile chassis. A first sensor is installed around the fifth wheel's axle to measure an actual vehicle velocity and a bar extending from the fifth wheel's axle is connected to the shaft. The system also includes a yawing sensor provided at one end of the bar near the shaft to measure a yaw motion of the automotive vehicle. The yawing sensor includes a light emitting element and a light-receiving element and cooperates with reflectors reflecting the light from the light emitting element onto the light receiving element.