Public mood is a key concept in explaining collective activity, but the way social media shapes an audience's public mood is still not fully understood. This study aims to explore how social media posts with various characteristics change public mood. The authors asked 351 participants to read 30 microblog newsletters with a 2 x 2 between-subject design (public x private sphere; positive x negative value). The results showed that (a) positive private information decreased negative public mood, (b) positive public information decreased positive public mood rather than increasing it, and (c) negative private information reduced the positive public mood of individuals who were high in justice sensitivity. The discussion focuses on the adverse effect of overexposure to positive public information and how individuals' means of information processing vary.