The numerical values of swimming speed and the change of form of fish school were calculated. The individuals kept uniform shape in a regular manner with the same space and direction as long as the speed was comparably fast. However, when the speed showed a slow down, the schooling form became irregular. The relationship between these two factors were discussed by means of cross-correlation coefficient analysis. The modification of schooling form had occured 0.75 seconds before the beginning of their speed change. The time lag was considered to be the latent period of the species that were used in our experiment.