In this paper, we propose a new method that can estimate a normal force and finger posture for touch panel devices by measuring transmitted light on fingernail. When a user touches a light source with their finger, the light (mainly red component) can be seen at a fingernail since the red light can go through the finger tissue. Based on this characteristic, we can estimate the manipulatory force by detecting the light intensity at the fingernail because the intensity of the transmitted light increases according to the applied force. We can also estimate the relative angles of the finger to a touch panel device. Even though the transmission through the finger tissue is diffusive, we can know the movement of the light source under the finger with a camera attached onto the fingernail. Therefore, a transient trajectory pattern of the light source is useful for estimating the relative direction between the finger and the device. When the posture of the finger is estimated, we can use “rotate” motion for manipulation. Our proposed method can be used as a new input device.