To provide an apparatus by which a water-absorbing resin that is homogeneous, contains only small amounts of residual monomers and water-solubles, and has an excellent water-absorbing ability with a high water-absorbing ratio is continuously manufactured without using troublesome operation and expensive equipment.

The outer circumference of the cylindrical rotor 2 of a polymerization machine is divided with barriers 5 to form open boxes 13. When an open box 13 comes on the top of rotor 2 as it rotates, a liquid monomer mixture is fed into the open box 13 in which polymerization takes place. When open box 13 comes at the bottom of rotor 2 and the opening of the open box 13 faces downward, the polymer gel is taken out, as it drops naturally by its own weight.