In desperation to learn the details of their case and to achieve the right to know, the families of crime victims in Japan have been taking action to implement the victim participant system in criminal trials. Focusing on the victim participant system, which began in 2008, the present study examined whether or not family members of victims who actually participated in criminal trials felt that their demands to know were fulfilled through the process. We administered a survey targeting 173 individuals whose family member has been the victim of a crime such as murder. The results revealed that their demands were fulfilled as they had expected only when they participated in the trial. Furthermore, we found that such sufficiency level led to their satisfaction with the justice system, while insufficient fulfillment of demands led to an increase in people’s willingness to act toward changing the system. We discussed the social consequences of judicially guaranteeing a victim’s right to know.