1. An image processing method for detecting a target, the image processing method comprising: acquiring depth information of an image; creating a histogram on the depth information of the image; setting a critical value of the depth information for detecting a region of a detection object from the image; extracting a region of the detection object from the image by using the set critical value of the depth information; verifying whether the extracted region of the detection object corresponds to the target; storing the extracted region of the detection object; and regulating the set critical value when it is determined that the extracted region of the detection object does not correspond to the target, wherein the extracting, the verifying, and the storing are repeatedly performed based on the regulated critical value.