Over the past few years, the robot industry has evolved rapidly and collaboration between the industrial robot and human has increased. From this point of view, many studies about robot safety index have been conducted recently. And most of those studies, only single factor among many important ones was dealt with per paper as an index of safety. But single quantity cannot capture the complex behavior of human body parts. In this paper, macroscopic and microscopic observation and injury classification with medical-oriented analysis are considered as safety criteria. Microscopic observation is conducted by microscope with hematoxylin and eosin stained tissue specimen and those results are analyzed with newly developed injury classification. Indexing is derived from the data of pig skin impact experiment where we focused on the effect of the impact (or collision) velocity, the impactor (a model of the robot which would collide with the human) mass and the impactor fillet radius. Consequently, the limitations of velocity depending on the impact conditions are presented.