The present invention provides a method for producing, by adding an additive and/or gas bubbles to a monomer with high efficiency, a polyacrylic acid (salt)-based water-absorbing resin having high physical properties while high productivity is maintained. The step of preparing an aqueous monomer solution includes the steps of: preparing an aqueous solution; and adding a water-insoluble additive and/or gas bubbles. A retention time from when the water-insoluble additive and/or the gas bubbles is/are added to when polymerization starts is 1 second to 60 seconds.