To obtain an active ester compound having a group constituted of a diamondoid structure.

This active ester compound having the group constituted of the diamondoid structure is represented by formula(2)( wherein, R is an alkyl, an aromatic group, a pyridyl, a quinolyl, succinimido, maleimido, a benzoxazole group, a benzothiazole group or a benzotriazole group, wherein hydrogen atoms in these groups may be substituted with at least one organic group; X is a group constituted of diamondoid structure; and (n) is an integer of 1-10 ).