1. A system for detecting an object by using depth information, comprising: a depth image receiver that receives a depth image from a depth camera; an object characteristic extractor that extracts based on a center depth value of the depth image, a strong classifier that classifies an object region and a non-object region in the depth image based on the characteristic of an object; and an object detector that detects the classified object region, wherein the strong classifier comprises a plurality of weak classifiers which are cascade connected to each other and classifies the object region and the non-object region by passing the depth image through the weak classifiers, and the plurality of the weak classifiers are generated through a training process for classifying positive training images among a multiple number of positive training images and a multiple number of negative training images.