The present invention provides a collagen crosslinking agent superior in biocompatibility that is free from the damage by UV irradiation and also from the problems of toxicity caused by residual monomer or unreacted functional groups. Provided is a noncovalent collagen crosslinking agent (for fibrous protein collagen), comprising a spacer of a polyvalent alcohol having two or more OH groups at the terminals and arms of collagen peptides formed of repetitions of three amino acids, the arms being bound via the OH groups to the spacer.