Overlap was found between habitats of Helice tridens, primarily an occupant of the upper and lower part of the intertidal areas between MTL (mean tidal level) and MHWS (mean high water spring), and H. japonica, usually restricted to the lower part of the intertidal area of the Shigenobu River Estuary, in southwest Japan. While immature individuals and large-sized adults of H. tridens occurred over the intertidal areas, the young adults of H. tridens were found only in the upper part of the intertidal area. This difference in the pattern of distribution between the young adults and the immature individuals of H. tridens seemed to be derived from interspecific competition or intraguild predation between H. tridens and H. japonica and/or intraspecific interactions within each species. These two species have the same of detritivore and carnivore food habits, and conspecific predation within and/or intraguild reciprocal predation between them were frequently observed in the field. In the present study, we performed field caging experiments to examine the dependence of predation ability on size between the two species. When individuals were ≦7.0 mm carapace width (CW), H. japonica had a higher ability to prey on members of H. tridens of the same size than H. tridens had on those of H. japonica. When individuals were ≧8.0 mm CW, there was no difference between the two species in terms of predatory ability, and the predation rate itself decreased. Therefore, H. japonica can exclude H. tridens from the lower habitats. However, the large sized individuals of H. tridens may intrude into the lower part of the intertidal area from the upper part. They can be against H. japonica, and can prey on the small sized individuals of H. japonica. However, thick sediment set in the cages increased the survival rate of the small individuals of H. japonica, because burrows dug in the sediment could be refuges for those of H. japonica from the intraguild predation by the large sized individuals of H. tridens. These refuges for the small individuals of H. japonica can be refuges also for those of H. tridens. However, burrows cannot be refuges from intraguild predation between similar-sized individuals of two species of the genus Helice. We can conclude that intraguild predation by H. japonica excluded the immature individuals of H. tridens in the lower habitat and restricted the young adults of H. tridens to the upper habitats in the estuary.