In most study of collective behavior, many numerical models have been developed. However, there is little field data collected for comparison with the models. We observed soldier crabs in Iriomote Island, Japan, which live in tideland forming big swarms, and analyzed their collective motion by transforming their positions on two-dimensional coordinates. The results of analysis show that the fluctuation of their motion positively contributes to maintenance and/or formation of a swarm, in contrast to the fluctuation of classical model, which is given as just random noise.