We propose a head-free, easy-setup gaze tracking system designed for a gaze-based Human-Computer Interaction. Our system enables the user to interact with the computer soon after catching the user's eye blinks. The user can move his/her head freely since the system keeps tracking the user's eye. In addition, our system only needs a 10 second calibration procedure at the very first time of use. An eye tracking method based on our unique eye blink detection and a sophisticated gaze estimation method using the geometrical eyeball model realize these advantages. Keywords gaze tracking, eye movement, eyeball model, eye tracking, eye blink detection.