Based on the spiral of silence theory, this study drew the motivational factors of SNS users' opinions on 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, self-censorship and self-monitoring as motive factors of opinion expression, and examined the differences in the effect of self-censorship and self-monitoring on willingness to speak out. 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. In addition, fear of negative evaluation showed a positive impact on self-censorship and self-monitoring, but the path of each of the two variables to the willingness to speak out showed different results—the former showed a negative impact, and the latter showed positive effects. This study has found a theoretical limitation that has been overlooked in the spiral of silence theory and suggests a new theoretical framework to reveal the process of public opinion formation. Furthermore, we applied this to the social media environment and empirically verified the specific causal mechanism of the individual opinion process.