In a hiyarihatto activity, workers report and discuss incident cases related to their work. It is particularly effective for cultivating participants’ attitudes about safety. Nevertheless, conventional face-to-face hiyarihatto activity includes features that are inappropriate for conduct in nuclear power plant organization. For example, workers at nuclear power plants are geographically distributed and busy. Therefore, they have great difficulty in participating in face-to-face hiyarihatto activity. Furthermore, workers’ hesitation in discussing problems inhibits continuation of their active participation. Purposes of this study are proposal of a hiyarihatto activity with asynchronous and distributed Computer Mediated Communication (CMC) for nuclear power plant organization, with demonstration of its effectiveness through field practice. The proposed method also features the introduction of special participants who follow action guidelines for promotion of the continuation of the activity. The method was practiced in an actual nuclear power plant organization. Results showed that the method is effective in some conditions, such as during periods without facility inspection. Special participants promoted the activity in some cases. Moreover, other factors affecting the activity and some improvements were identified.