In this research, we attempted to evaluate the cyclist's ability to maintain balance by using information from a 9-axis motion sensor attached to the cyclist's trunk, lower limbs, and bicycle body. The experiment is conducted on flat and uneven roads. First, the sensor fusion using a gyro sensor, an acceleration sensor, and a magnetometer is used to estimate the trunk posture during riding a bicycle. Then, we evaluate the coordination of acceleration between the rider’s trunk and the bicycle by using the singular value decomposition. Finally, the results of the three-dimensional posture of the trunk and the coordination of acceleration between trunk and the bicycle are used to verify the balance during riding a bicycle.