Interleukin-17 (IL-17) has been suggested to have a protective effect on the intestinal mucosa. The administration of an anti-IL-17 receptor monoclonal antibody has been associated with the onset of inflammatory bowel disease. We present a case of ulcerative colitis caused by secukinumab, an anti-IL-17 receptor A monoclonal antibody, that was treated with mirikizumab, a p19-directed antibody against IL-23. A man in his 20 s with psoriasis vulgaris was administered secukinumab at the dermatology department of our hospital. After the induction of secukinumab therapy, he gradually developed diarrhea, and after 4 months, bloody stools were observed. The patient was diagnosed with left-sided ulcerative colitis based on the findings of a colonoscopic examination. Secukinumab was discontinued and prednisolone was started; however, there was little improvement. His symptoms improved with the introduction of mirikizumab, and the condition is now clinically and endoscopically stable. In this case, an anti-IL-23 antibody was effective against ulcerative colitis induced by an Il-17A antibody agent. Although IL-23 exists upstream of the Th17 system, mirikizumab, an anti-IL-23 antibody preparation, has been shown to have the potential to alleviate UC.