Skin reactions to nivolumab are typical immune-related adverse events. We investigated the relation between patient background and test values before nivolumab administration and skin reactions. From February 2016 to February 2017, we evaluated the clinical outcomes of 21 patients who were administered nivolumab. Patients were divided into 2 groups: 3 cases of skin reactions to nivolumab(skin reaction group)and 18 cases without skin reactions to nivolumab(non-skin reaction group). In the skin reaction group, the numbers of eosinophils and basophils before nivolumab administration were significantly higher than those in the non-skin reaction group(p=0.0015 and p=0.0075, respectively). It was suggested that the numbers of eosinophils or basophils before nivolumab administration might be associated with the appearance of skin reactions.