A process for producing a polyoxyalkylene derivative substituted with succinimidyl group which comprises reacting a polyoxyalkylene compound having carboxyl group at ends with N-hydroxysuccinimide in an inert solvent in the presence of dicyclohexylcarbodiimide; filtering an obtained reaction product; and crystallizing the polyoxyalkylene derivative substituted with succinimidyl group by adding isopropyl alcohol to a filtrate obtained by the filtration, an amount by weight of isopropyl alcohol being 1 to 100 times as much as an amount by weight of the polyoxyalkylene compound having carboxyl group at ends. Materials can be handled with safety in the process of the present invention, and a highly pure polyoxyalkylene derivative substituted with succinimidyl group which does not cause turbidity in an aqueous solution and has an excellent quality as a material for drugs can be produced.