An abnormal traffic detection apparatus and method based on Modbus communication pattern learning is provided. The abnormal traffic detection apparatus based on the Modbus communication pattern learning previously detects and responds to abnormal traffic on a Modbus/TCP protocol. According to the present invention, a communication service between control systems can be stably provided by previously detecting the abnormal traffic capable of interfering with a stable operation of the control system. Particularly, since the effective abnormal traffic on the Modbus/TCP protocol can be previously detected, security of the control system can be increased by rapid detection and response with respect to security threats on the Intranet of the control system, and availability can be secured.