1. A head mounted device (HMD) comprising: at least one camera configured to capture one or more partial region images of an iris; an electrocardiogram (ECG) sensor configured to receive ECG signals; and at least one processor configured to authenticate a user by acquiring at least one of the one or more partial region images, which are captured by the at least one camera, and the ECG signals, which are received through a plurality of electrodes, wherein the at least one processor is further configured to generate a normalized image to authenticate the user, by combining the one or more partial region images and by using a curvature of an inner contour line of the iris, which is included in the one or more partial region images.