When bullying occurs in school, most students are not only aware of it, but are also present and witness bullying. A bystander may recognize bullying as wrong, and feel responsible and motivated to intervene, but others may occasionally fail to actively intervene and defend a bullied peer for any number of reasons. The purpose of this study was to investigate how moral distress, peer norms and collective moral disengagement were related to bystander behaviors (assertive defender, comforting defender, sympathetic outsider, and indifferent outsider) in bullying. In addition to examining the pathways to bystander behaviors, we also tested the hypothesis that middle school students have different pathways from those of primary school students. One thousand one hundred Japanese primary school students (M=10.6 years, 48.5% girls) and one thousand four hundred and fifty-five middle school students (M=13.6 years, 49.9% girls) participated in our questionnaire survey. T-test showed that compared with middle school students, primary school students significantly expressed more defending, higher peer norms, and lower collective moral disengagement. In order to examine how each variable is related to the occurrence of bystander behaviors, we converted the experience of bystander behaviors into a binary value, with "never" set to "0" and all other responses set to "1", and conducted a binomial logistic regression analysis by using experience of bystander behaviors as the criterion variable. Results showed that bystander behaviors were positively predicted by collective moral disengagement and that outsider behaviors were negatively predicted by peer norms. Moreover, the results of simple slope analysis for the significant effect of two-way interaction suggested that peer norms might regulate the effect of moral distress on defending behaviors among primary school students, and that collective moral disengagement might regulate the effect of moral distress on outsider behaviors among middle school students. Implications of these findings are discussed.