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), polyvinyl alcohol (C2) in an amount of more than 0% by weight but not more than 50% by weight relative to a total amount of the ethylenically unsaturated group-containing monomers (A), having a degree of saponification of more than 91.5 mol % and less than 98 mol %, and water. There is also provided a sustained release preparation including the water dispersion and a volatile active substance which is selected from a group consisting of a pheromone substance, an agricultural chemical, an aromatic, a deodorant and an antibacterial agent.
