Network traffic monitoring has long been a core element for effective network management and security. However, it is still a challenging task with a high degree of complexity for comprehensive analysis when considering multiple variables and ever-increasing traffic volumes to monitor. For example, one of the widely considered approaches is to scrutinize probabilistic distributions, but it poses a scalability concern and multivariate analysis is not generally supported due to the exponential increase of the complexity. In this work, we propose a novel method for network traffic monitoring based on clustering, one of the powerful deep-learning techniques. We show that the new approach enables us to recognize clustered results as patterns representing the network states, which can then be utilized to evaluate "similarity" of network states over time. In addition, we define a new quantitative measure for the similarity between two compared network states observed in different time windows, as a supportive means for intuitive analysis. Finally, we demonstrate the clustering-based network monitoring with public traffic traces, and show that the proposed approach using the clustering method has a great opportunity for feasible, costeffective network monitoring.