The entropy of microscopic configurations of a classical ideal gas system located on a one-dimensional ring is investigated. It is verified that the distribution of particles on the ring becomes more and more uniform，and the positional entropy increases with time toward the maximum value，while the total entropy (contributed by positional uncertainty and velocity uncertainty) is conserved in the rigorous mathematical sense. The velocities of particles which are passing through the same position at the same time become more and more quantized，and the gap between adjacent discrete velocity values gradually reduces to zero as time increases. This quantization property means that，as the velocity resolution limit of an observer must be finite in the sense of physical measurement，the measured entropy of microscopic configurations must be increasing with time. This simple system is helpful for understanding the second law of thermodynamics.