An apparatus (2) includes a neutralization tank (3), a pump (4), a heat exchanger (6), a line mixer (8), a polymerizer (10), a first pipe (12), a second pipe (14), a third pipe (16), a fourth pipe (18), and a fifth pipe (20). Continuously supplied into the neutralization tank (3) are a monomer aqueous solution and a basic aqueous solution, so as to prepare a mixture solution. The mixture solution is circulated through the first pipe (12), the pump (4), the second pipe (14), the heat exchanger (6), and the third pipe (16). The mixture solution is supplied to the polymerizer through the fourth pipe (18). The apparatus (2) satisfies a requirement that a value X is not more than 200, which value X is found according to the following expression: “X=(V/F)·A”, wherein V is a volume of the mixture solution present in a neutralization system, F is a flow volume of the mixture solution to be supplied to a polymerization system, and A is a contact area of the mixture solution present in the neutralization system with a device(s) and a pipe(s) constituting the neutralization system. With this arrangement, it is possible to provide a water absorbent resin having superior whiteness.