Based on the spiral of silence and impression management theory, this study drew the motivational factors of SNS users" opinion for social discourse and examined the processes by which individuals make social voices or become silent in social media according to the climate of opinion. This study suggested perceived opinion support, fear of negative evaluation, and SNS self-presentation tendency as motive factors of opinion expression. To verify this research model, data were collected from adults using SNS in Korea, and we conducted analysis by PLS (Partial Least Square) technique. The empirical analysis results revealed that the perceived opinion support had a negative effect on the fear of negative evaluation and the fear of negative evaluation showed negative influence on the willingness to speak out. On the other hand, unlike the hypothesis presented in this study, the fear of negative evaluation showed positive influence on SNS self-presentation tendency. Finally, SNS self-presentation tendency had a positive influence on willingness to speak out. As a result, there was a competitive mediation in which the sign of the direct path leading to the willingness to speak out and the sign of the indirect path through SNS self-presentation tendency were opposite to each other. Further analysis revealed that the SNS self-presentation tendency acted as a suppression variable in the structural model. As the SNS self-presentation tendency variable was included and controlled in the structural model, the effect of the direct path from the fear of negative evaluation to the willingness to speak out was strengthened. This study found a theoretical limitation that had been overlooked in the spiral of silence theory and suggested a new theoretical framework to reveal the process of public opinion formation using theoretical combination with impression management theory. Furthermore, we applied this to the social media environment and empirically verified the specific causal mechanism of the individual opinion process.