The present invention has objects to provide a mutant interferon-α protein having a higher anti-viral and anti-tumor activity and to provide an agent for susceptive diseases, which contains the mutant interferon-α protein as an effective ingredient; and solves the above objects by providing a mutant protein which has an amino acid sequence of human interferon-α subtype α8 represented by any one of SEQ ID NOs:1 to 3, where the arginine residue at the 145th has been replaced with leucine, isoleucine, or valine residue; the alanine residue at the 146th has been replaced with asparagine or serine residue; and the methionine residue at the 149th has been replaced with tyrosine residue; and an agent for susceptive diseases, containing the mutant protein.