PURPOSE: An aqueous dispersion of a substituted succinic anhydride, containing a specific water-soluble copolymer and having improved sizing effect and good dilution stability in use as a sizing agent utilized in a paper making process.

CONSTITUTION: An aqueous dispersion obtained by adding (B) a mixture prepared by additionally premixing a substituted succinic anhydride with an oil-in- water type surfactant to (A) (A1) a water-soluble copolymer containing 50W98.5mol% (meth)acrylamide, 1W49.5mol% N,N-dialkylaminoalkyl (meth) acrylamide and 0.5W20mol% (meth)acrylic acid as monomer components or (A2) a water-soluble copolymer consisting of ≥70wt% mixed monomer components of the component (A1) and another monomer copolymerizable monomer or an aqueous solution containing both components (A1) and (A2) and preparing a dispersion in a mixer. The aimed stable dispersion with 0.01W25wt% content of the component (B) is obtained by using 0.2W2pts.wt., based on 1pt.wt. component (B), component (A).