To efficiently produce a water absorbing composite having good water absorbency without requiring a complicated step of removing residual monomers by completing polymerization in a short time.

A monomer aqueous solution containing a water-soluble acrylic monomer affording a water absorbing polymer by the polymerization, a peroxide and a transition metal ion is applied to a fibrous substrate. The resultant fibrous substrate is then irradiated with ultraviolet rays to polymerize the water-soluble acrylic monomer.