Most injury is caused by an external cause to the human body, resulting in morphological or functional disability. However, in the physical impairment related diseases, it is difficult for the disease to occur only by internal factors, and it can be said that internal factors and external factors are combined. Therefore, when the physical impairment is caused by disease, it is important to the apportionment analysis of injury to think about the cause of the physical impairment by considering both the external and internal factors. The most commonly used the apportionment guide is the “Rhim's trauma apportionment guideline” in impairment evaluation. But this guideline is often difficult to assess in certain diseases. Since the apportionment analysis of external factors can bring about a difference in degree according to the viewpoint of the assessor, the degree of the physical impairment may vary greatly depending on case by case. Especially, it is not reasonable to assess based on causal relation with common sense or emotional, and it is necessary to make rational assessment based on medical evidence and professional knowledge.