1. A system for monitoring a queue, the system comprising a processor, wherein said processor configured to: receive image information including depth information of objects in the queue and a background of a photographing space from a depth detection device having a sensor, via a network; calculate actual heights of the objects using the depth information of the objects and the background obtained from the image information and initial parameters including a height and an angle of the depth detection device measured by the sensor; create cluster set information composed of a plurality of cluster information calculated by filtering the objects in the image information with a plurality of different height values; and identify each of the objects by comparing the cluster set information with a predetermined pattern; assign waiting time information to the identified each object in order to count waiting time of the identified each object when the each object is identified; and track the identified each object in order of the assigned waiting time information, wherein the cluster information is a set of pixels obtained for one height value and the cluster set information is created by three-dimensionally accumulating sets of pixels corresponding to the cluster information in accordance with height values, and wherein when cluster information obtained for different height values are overlapped, said processor is configured to remove cluster information obtained for a lower height value among the overlapped cluster information.