To obtain a complex of an organic peroxide or an azo-based polymerization initiator, capable of improving disperse stability of the organic peroxide in water and improving safety of workers, unnecessary to use a carrier, etc., and useful as an initiator for graft polymerization by including the organic peroxide into a clathrate compound.

(B) An organic peroxide such as benzoyl peroxide is included in (A) a clathrate compound which is preferably a cyclodextrin compound or calixarene compound. A mixing molar ratio of the component B to the component A is preferably about 1. The complex is obtained by dissolving the component B in a monomer used for graft polymerization, adding the component A thereto, further adding water thereto and stirring these components. Formation of a homopolymer as a by-product is suppressed and hydrophilification of fiber structure is made possible by carrying out graft polymerization processing of fiber structure by using the complex.