The invention provides a diamine crosslinking agent for acidic polysaccharides consisting of a diamine compound having a primary amino group at both terminals and an ester or thioester bond in the molecule, wherein the number of atom in the linear chain between at least one of the amino groups and the carbonyl carbon in the ester or thioester is 1 to 5; in particular, a diamine crosslinking agent for acidic polysaccharides which is represented by the general formula (I) below: [the symbols in the formula are as described in the specification]; a crosslinked acidic polysaccharide obtained by forming crosslinks by amide bonding between the amino groups in the diamine crosslinking agent and carboxyl groups in an acidic polysaccharide; and a medical material including the crosslinked product.