There is provided a water dispersion for a sustained release preparation, the dispersion having viscosity at 25° C. of not more than 100 mPa·s and including polymer particles which are obtained by polymerizing ethylenically unsaturated group-containing monomers (A) at least one kind of a hydrophilic substance (B) in an amount of more than 0% by weight but not more than 20% by weight relative to a total amount of the ethylenically unsaturated group-containing monomers (A), being selected from a group consisting of a surfactant, a plasticizer and a moisturizer polyvinyl alcohol (C) in an amount of more than 0% by weight but not more than 30% by weight relative to a total amount of the ethylenically unsaturated group-containing monomers (A), having a degree of saponification of more than 82 mol % and water.
