A steel plate comprising: C: 0.02 to 0.15% by mass, Si: 0.02 to 0.50% by mass, Mn: 0.6 to 2.0% by mass, P: more than 0% by mass and 0.030% by mass or less, S: more than 0% by mass and 0.003% by mass or less, Al: 0.010 to 0.080% by mass, Ca: 0.0003 to 0.0060% by mass, N: 0.001 to 0.01% by mass, and O: more than 0% by mass and 0.0045% by mass or less, optionally comprising one or more elements selected from the group consisting of: B: more than 0% by mass and 0.005% by mass or less, V: more than 0% by mass and 0.1% by mass or less, Cu: more than 0% by mass and 1.5% by mass or less, Ni: more than 0% by mass and 1.5% by mass or less, Cr: more than 0% by mass and 1.5% by mass or less, Mo: more than 0% by mass and 1.5% by mass or less, Nb: more than 0% by mass and 0.06% by mass or less, Ti: more than 0% by mass and 0.03% by mass or less, Mg: more than 0% by mass and 0.01% by mass or less, REM: more than 0% by mass and 0.02% by mass or less, and Zr: more than 0% by mass and 0.010% by mass or less, with the balance being iron and inevitable impurities, wherein the steel plate satisfies the following formulae and: 3.0 ≤ Ca / S Ca − 1.25 × S / O ≤ 1.80 where [Ca], and are contents in % by mass of Ca, S and O respectively, and wherein an area ratio of a part that has a defect echo height of 20% or more is 0.05% or less, the defect echo height means a ratio in % of an intensity of a defect echo reflected by a defect inside a test piece taken out from a part of the steel plate to an intensity of a bottom-surface echo reflected by the bottom surface of the steel plate or test piece, and the defect echo height is measured in accordance with the method specified in the description. C: 0.02 to 0.15% by mass, Si: 0.02 to 0.50% by mass, Mn: 0.6 to 2.0% by mass, P: more than 0% by mass and 0.030% by mass or less, S: more than 0% by mass and 0.003% by mass or less, Al: 0.010 to 0.080% by mass, Ca: 0.0003 to 0.0060% by mass, N: 0.001 to 0.01% by mass, and O: more than 0% by mass and 0.0045% by mass or less, optionally comprising one or more elements selected from the group consisting of: B: more than 0% by mass and 0.005% by mass or less, V: more than 0% by mass and 0.1% by mass or less, Cu: more than 0% by mass and 1.5% by mass or less, Ni: more than 0% by mass and 1.5% by mass or less, Cr: more than 0% by mass and 1.5% by mass or less, Mo: more than 0% by mass and 1.5% by mass or less, Nb: more than 0% by mass and 0.06% by mass or less, Ti: more than 0% by mass and 0.03% by mass or less, Mg: more than 0% by mass and 0.01% by mass or less, REM: more than 0% by mass and 0.02% by mass or less, and Zr: more than 0% by mass and 0.010% by mass or less, with the balance being iron and inevitable impurities, wherein the steel plate satisfies the following formulae and: 3.0 ≤ Ca / S Ca − 1.25 × S / O ≤ 1.80 B: more than 0% by mass and 0.005% by mass or less, V: more than 0% by mass and 0.1% by mass or less, Cu: more than 0% by mass and 1.5% by mass or less, Ni: more than 0% by mass and 1.5% by mass or less, Cr: more than 0% by mass and 1.5% by mass or less, Mo: more than 0% by mass and 1.5% by mass or less, Nb: more than 0% by mass and 0.06% by mass or less, Ti: more than 0% by mass and 0.03% by mass or less, Mg: more than 0% by mass and 0.01% by mass or less, REM: more than 0% by mass and 0.02% by mass or less, and Zr: more than 0% by mass and 0.010% by mass or less, with the balance being iron and inevitable impurities, wherein the steel plate satisfies the following formulae and: 3.0 ≤ Ca / S Ca − 1.25 × S / O ≤ 1.80 where [Ca], and are contents in % by mass of Ca, S and O respectively, and wherein an area ratio of a part that has a defect echo height of 20% or more is 0.05% or less, the defect echo height means a ratio in % of an intensity of a defect echo reflected by a defect inside a test piece taken out from a part of the steel plate to an intensity of a bottom-surface echo reflected by the bottom surface of the steel plate or test piece, and the defect echo height is measured in accordance with the method specified in the description.