PURPOSE: The titled derivative that is obtained by reaction between a water- soluble polysaccharide with a spacer bonded and a mitomycin, thus being composed of a polysaccharide bonded through a spacer to a mitomycin and showing good transition to lymph and long activity duration.

CONSTITUTION: A water-soluble polysaccharide such as dextran is dissolved in water and a cyan halide is added thereto, while the pH is maintained to 10.7. Then, the activated polysaccharide is allowed to react with an amino-fatty acid such as epsilon-amino-n-caproic acid as a spacer near pH 9, and a mitomycin is allowed to react with the polysaccharide bearing spacers in the presence of a condensation agent such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide at a pH between 5 and 6 to give the objective substance.