A robot gripper is provided comprising two robot arms, upper contact parts, and lower contact parts disposed at the ends of each of the two robot arms. The upper and lower contact parts are in contact with a top and a bottom of an article when gripping the article, The upper and lower contact parts are semispherical shaped and have predetermined radii. Sensor units are mounted on the upper contact parts and the lower contact parts. The sensor units measure vertical or horizontal forces applied to the upper contact parts or the lower contact parts when gripping the article. A control unit configured to determine whether the center of gravity of the article is located at a center position between the ends of the arms using vertical and horizontal distances between the ends of the arms and vertical components of the forces measured by the sensor units when gripping the article is provided.