PURPOSE: To separate a steric isomer (especially racemic compound) of a β- lactam which is important as pharmaceuticals such as antibiotic substance and raw materials for other pharmaceuticals by using a specific polysaccharide derivative as a separation agent.

CONSTITUTION: Racemic β-lactam of formula III (R1 to R4 are H or 1-30C substituent and at least one of the combinations R1 and R2, and R3 and R4 is combination of different substituents; R5 is 1-30C substituent or silyl having 1-30C substituent), formula IV (R6 is 2-50C bivalent substituent), formula V (R7 is H or R5; R8 is R6) or formula VI can be separated by using a separation agent composed mainly of a polysaccharide derivative produced by substituting a part or total of hydrogen atoms on hydroxyl groups or amino groups of a polysaccharide with one or more kinds of atomic groups of formula I, formula II or R (R is 1-30C group having at least one asymmetric center) [e.g. amylose (S)- methylbenzylcarbamate].