Structured arguments are commonly used to communicate to stakeholders that safety, security or other attributes of a system are achieved. Due to the growing complexity of systems, more uncertainties appear and the confidence in arguments tends to be less justifiable by reviewing. In this paper, we propose a quantitative method to assess the confidence in structured arguments, like safety cases. We adopt the Goal Structuring Notation (GSN) to model the safety case and propose to add annotations to identify uncertainties in this model. Three inference types of arguments are proposed according to their impact on confidence. Definition and quantification assessment of confidence are based on the belief function theory. The proposed approach is illustrated with several GSN examples.