A process for producing a water absorbent resin is provided with which it is possible to efficiently and inexpensively obtain a surface-crosslinked water-absorbing resin having excellent material properties, while ensuring high productivity. In continuous production scaled up to a large scale (in particular, 1 t/hr or more), the physical properties are improved and stabilized (for example, a reduction in standard deviation of the physical properties) by a surface crosslinking treatment to further improve absorption against pressure (AAP) and saline flow conductivity (SFC). The process for producing a polyacrylic acid (salt)-type water absorbent resin includes a surface treatment step in which after addition of a surface-crosslinking agent, a crosslinking reaction is conducted in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having with a plurality of stirring discs, and heating means, at a stirring-power index of the device of 3-15 W·hr/kg.(Stirring-power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling))×average retention time)/(treatment amount per unit time×average retention time).
