A low cost, a disposable diaper is provided that presents no difficulties in disposable diaper manufacture in humid climates, and that has a minimal amount of flowback, and rapid absorption time. A particulate water-absorbing agent is provided having as the principal component a poly (meth)acrylic acid (salt)-based water absorbing resin, wherein the water-absorbing agent has a particle size distribution of a weight average particle diameter of 300 to 500 μm, a blocking ratio after moisture absorption when left for one hour at 25° C. and 90% relative humidity of 20% or less, a surface tension of 60 mN/m or more, and a gel capillary absorption (GCA) of 28.0 g/g or more.