PURPOSE: To obtain a reactive microsphere, comprising a specific copolymer, excellent in dispersion stability in water or an organic medium, capable of readily and stably immobilizing a functional substance such as a protein and hardly causing the nonspecific adsorption of the protein, etc.

CONSTITUTION: This microsphere comprises a copolymer of a (poly)oxyalkylene derivative of the formula [OA is a 2-4C oxyalkylene; (n) is the average number of added mol of OA and is 1-1000; R1 and R2 are each H or CH3] and a hydrophobic radically polymerizable monomer (preferably styrene). The microsphere is preferably obtained by polymerizing a compound of the formula with the radically polymerizable monomer in water or a mixed solvent of the water with ethanol in the presence of a radical polymerization initiator at 10-60°C for 1-24hr. Furthermore, the compound of the formula is obtained by reacting, e.g. a mono(meth)acrylic ester of a (poly)alkylene glycol with N,N'- carbonyldiimidazole.