The present invention intends to provide a novel monoclonal antibody having a binding specificity to GALP or its derivative, which is useful in developing therapeutic agents, preventive agents or diagnostic agent for diseases associated with GALP or its derivative, and a method of quantifying GALP using the antibody. More specifically, the present invention provides an antibody specifically reacting with a partial peptide in the C-terminal region of a polypeptide having the amino acid sequence represented by SEQ ID NO: 1, SEQ ID NO: 2 or SEQ ID NO: 3, or its derivative, and a method of quantifying GALP or its derivative.