The crosslinking agents and condensing agents that have been employed in biological adhesives and in treating medical devices such as cardiac valves are non-natural compounds synthesized artificially. Thus, they are not metabolized in vivo and exhibit toxicity to living bodies. These compounds are thus used only in a restricted amount and for limited purposes in the clinical sites. The present invention provides a biological low-molecular-weight derivative obtained by modifying carboxyl groups of a biological low-molecular-weight compound with N-hydroxysuccinimide, N-hydroxysulfosuccinimide, or a derivative thereof and a crosslinked high-molecular-weight product obtained by crosslinking various high-molecular-weight compounds with this derivative.