A steel plate for wind power generation plants, which has a predetermined chemical composition, and in which, when a plate thickness is set as t in units of mm, a microstructure at a depth position of t/4 from the surface is composed of, in area proportion, ferrite: 50 to 80%, bainite: 20 to 50% and other structures: 0 to 1.0%, wherein an average crystal grain size of the ferrite is 25.0 to 50.0 µm, wherein the porosity volume at a depth position of t/2 from the surface is 0.20×10 -3 cm 3 /g or less, and wherein the plate thickness is 60 mm or more and the plate width is 3,000 mm or more.