PROBLEM TO BE SOLVED: To provide a preventive/improving agent of a neurodegenerative disease developing due to accumulation of amyloid &bgr peptide fibril.SOLUTION: This invention relates to a preventive/improving agent of a neurodegenerative disease with formation of amyloid fibril, which is a preventive/improving agent, especially in which the neurodegenerative disease is Alzheimers disease or Parkinsons disease, concerning a preventive/improving agent containing a nucleoprotein as an active ingredient.