The assassin bug Agriosphodrus dohrni (Signoret) has gergarious nymphal stages, while nymphal cannibalism often occurs. Such cannibalism, however, would reduce the fitness among the kins hatched simultaneously, so that they might have adaptive tactics avoid such cannnibalism. As the nymphs aggregated arrestant at sites where oviposition took place, we hypothesized that the egg mass shell (EMS) would have some effect on avoiding nymphal cannibalism. To tested this hypothesis, frequency of the nymphal cannibalism was compared under conditions with and without the existence on the EMS. All those egg masses were ovipoisted by 27 adult female. Ten kin nymphs were kept together in a container with a piece of (1) EMS that they hatched from, (2) foreign EMS, (3) no EMS, (4) EMS already extracted with hexane and (5) EMS that was once extracted but retreated with the extract. Except for (3) and (4), more than 80 % of the nymphs survived to be 2nd-instar. These suggest that EMS would be involved in the suppressive effect on nymphal cannibalism. In addition, as extracted (4) did not suppress the nympal cannibalism, it is most likely that chemical substances in EMS would be effective to avoid nymphal cannibalism in the assassin bug.