An apparatus (2) includes: a first tank (4) a line mixer (6) a second tank (8) a cooling jacket (10) a first tube (12) a second tube (14) and a third tube (16). In the first tank (4), (i) sodium persulfate serving as a polymerization initiator and (ii) water are fed so as to obtained an aqueous solution. The aqueous solution is continuously taken out and transferred via the first tube (12) to the line mixer (6). Between the first tank (4) and the line mixer (6), water is injected into the first tube (12). The water and the sodium persulfate aqueous solution are stirred together in the line mixer (6). This stirring decreases a concentration of the aqueous solution. The resulting aqueous solution is continuously fed to the second tank (8) via the second tube (14). The aqueous solution is then continuously taken out via the third tube (16) and is continuously added to an acrylic acid (salt) aqueous solution. This provides a production method which allows a high-quality water absorbing agent to be obtained with a compact apparatus.
