The second colonoscopy showed diffuse granular lesions in the rectum and rough-surfaced mucosa with loss of vascularity from the rectum to the splenic flexure, as well as a cluster of micro-erosions around the appendiceal orifice. In the second histopathological examination, a decreased number of glandular ducts and goblet cells and diffuse infiltration of various inflammatory cells were observed. Based on these findings, the patient was diagnosed with left-sided ulcerative colitis. This report describes a case of ulcerative colitis progressing from the initial lesion to typical left-sided colitis. We also discuss differential diagnoses of rectal granular lesions.