1. A water level monitoring device which acquires image data including a monitoring area set to include a shore area and a water area and monitors a change in water level for the monitoring area, comprising: a receiving unit which receives the image data from an external device; an output unit which outputs a warning signal; a storage unit which stores a water level model with distinction between the water area and the shore area as similar characteristic areas; and a controller which sets at least one monitoring area, generates the water level model for the monitoring area in the image data received from the receiving unit, updates the water level model stored in the storage unit or accumulates the water level model in the storage unit, monitors a water level based on a change in the shore area and the water area of the water level model, and outputs a warning signal through the output unit, wherein, if a plurality of monitoring areas is preset, the controller generates water level models for the plurality of monitoring areas, and, if it is detected that a variation in pixels exceeds a reference value preset for one of the water level models, determines whether or not a variation in pixels for another water level model exceeds a preset reference value, and determines whether or not to output a warning signal, and wherein, if a plurality of monitoring areas and reference lines for the plurality of monitoring areas are set, if one of the water area and the shore area intrudes on a reference line set in one of the water level models, the controller determines whether or not a reference line set in another water level model is intruded, and determines whether or not to output a warning signal.