1. An apparatus for detecting a head image in an input image, comprising: a training DB storing therein a positive head image and a negative head image into which a plurality of images are classified; a sub-window processor extracting a feature population while sliding a sub-window of a predetermined size with respect to the input image in accordance with a plurality of instances; a head candidate region classifier classifying head candidate regions based on a head model learned from the extracted feature population from the sub-window processor by referring to samples stored in the training DB; and a head candidate region determiner determining a head candidate region as a head region by checking continuity of contours of the classified head candidate regions using the head model, wherein the contours are determined to be discontinuous when a degree of a transition between feature values of a checked head candidate region and feature values of the head model is equal to or larger than a predetermined value.