A forging steel ingot cast with a mold wherein: the number density D BOT of inclusions 5 to 10 μm in longest diameter observed on a steel cross-section is 10 to 80 pieces/cm 2 at the steel ingot lower portion ranging from the bottom end of the steel ingot to the bottom 20% of the total height of the steel ingot in the direction of the gravity; the number density D TOP of inclusions 5 to 10 μm in longest diameter observed on a steel cross-section is 20 to 90 pieces/cm 2 at the steel ingot upper portion ranging from the top end of the steel ingot to the top 20% of the total height of the steel ingot; the number density of inclusions 40 μm or more in longest diameter observed on a steel cross-section is 5 pieces/cm 2 or less at both of the steel ingot lower portion and said steel ingot upper portion; and the following expression is satisfied, D TOP / D BOT ≥ S / 18 here, represents the content (in mass ppm) of S in steel; and the forging steel ingot consists of C: 0.2 to 0.6% (in mass %, the same is applied hereunder), Si: 0.05 to 0.5%, Mn: 0.2 to 1.2%, Ni: 0.1 to 3.5%, Cr: 0.9 to 2.5%, Mo: 0.1 to 0.7%, V: 0.005 to 0.2%, Al: 0.01 to 0.1 %, S: 0.005% or less, Ti: 0.005% or less, and O: 0.0015% or less, and optionally one or more of B, W, Nb, Ta, Cu, Ce, Zr and Te in an amount of 0.1% or less, with the remainder composed of iron and unavoidable impurities, of which P is limited to 0.03% or less. the number density D BOT of inclusions 5 to 10 μm in longest diameter observed on a steel cross-section is 10 to 80 pieces/cm 2 at the steel ingot lower portion ranging from the bottom end of the steel ingot to the bottom 20% of the total height of the steel ingot in the direction of the gravity; the number density D TOP of inclusions 5 to 10 μm in longest diameter observed on a steel cross-section is 20 to 90 pieces/cm 2 at the steel ingot upper portion ranging from the top end of the steel ingot to the top 20% of the total height of the steel ingot; the number density of inclusions 40 μm or more in longest diameter observed on a steel cross-section is 5 pieces/cm 2 or less at both of the steel ingot lower portion and said steel ingot upper portion; and the following expression is satisfied, D TOP / D BOT ≥ S / 18 here, represents the content (in mass ppm) of S in steel; and the forging steel ingot consists of C: 0.2 to 0.6% (in mass %, the same is applied hereunder), Si: 0.05 to 0.5%, Mn: 0.2 to 1.2%, Ni: 0.1 to 3.5%, Cr: 0.9 to 2.5%, Mo: 0.1 to 0.7%, V: 0.005 to 0.2%, Al: 0.01 to 0.1 %, S: 0.005% or less, Ti: 0.005% or less, and O: 0.0015% or less, and optionally one or more of B, W, Nb, Ta, Cu, Ce, Zr and Te in an amount of 0.1% or less, with the remainder composed of iron and unavoidable impurities, of which P is limited to 0.03% or less. C: 0.2 to 0.6% (in mass %, the same is applied hereunder), Si: 0.05 to 0.5%, Mn: 0.2 to 1.2%, Ni: 0.1 to 3.5%, Cr: 0.9 to 2.5%, Mo: 0.1 to 0.7%, V: 0.005 to 0.2%, Al: 0.01 to 0.1 %, S: 0.005% or less, Ti: 0.005% or less, and O: 0.0015% or less, and optionally one or more of B, W, Nb, Ta, Cu, Ce, Zr and Te in an amount of 0.1% or less, with the remainder composed of iron and unavoidable impurities, of which P is limited to 0.03% or less.