An object is to provide a novel antibody or antigen-binding fragment thereof against exon 21 of human periostin. The object is achieved by an antibody or antigen-binding fragment thereof bindable to a conformational epitope on human periostin, wherein the epitope contains the amino acid sequence A represented by SEQ ID NO: 61 and the amino acid sequence B represented by any one of SEQ ID NOs: 62 to 64.