These days, Internet services realize computer-mediated social communication as social networking services (SNSs) such as WhatsApp, Facebook, Twitter, and so on, which expand worldwide. Through these SNSs, people express emotions by selecting phrases and characters. These emotional expressions are selected generally on the basis of the user’s subjective idea. However, when users lose self-control, they might send inappropriate expressions to others without proper consideration. To avoid such situations, some observable mechanisms that can detect and communicate unconscious emotions to users before they send the message are desired. Recently, some approaches to detecting unconscious emotions using biological sensors have been reported. These approaches provide procedures to make unconscious emotions observable and communicable to users in real time. We combine a biofeedback method with an unconscious emotion estimation method for detecting mismatch between an unconscious emotion and an outward expression before sending the message. Using this idea, we design and implement a system that detects such mismatch and gives feedback to an SNS user. We carry out an experiment using the proposed system. The results show that the system is helpful for the intended purpose but needs improvement.