The responsibility mental model (RMM) is an important factor in an effective cooperation system that affects decision-making, role adjustment, and performance. For this purpose, 186 firefighters from an off-site fire brigade and 65 firefighters from an on-site fire brigade participated in the study. We compared the levels of the RMM for a fire at a nuclear plant and analyzed whether the RMM is shared internally in each fire brigade through latent profile analysis. The results showed that the responsibility of on-site fire brigades is prioritized for fire liability in nuclear plants. Both brigades identified the heterogeneity of the RMM. Firefighters from the off-site fire brigade were divided into three groups of thinking they have ‘more responsibility’ (11%), ‘shared responsibility’ (36%), and ‘less responsibility’ (53%) than their cooperation partner. Firefighters from the on-site fire brigade were divided into two groups of ‘more responsibility’ (48%) and ‘shared responsibility’ (52%). The result that each fire brigade does not share a homogeneous RMM suggests the ambiguity of roles in the cooperation system. The need for infrastructure for cross-organizational collaboration and coordination is discussed.