A steel plate comprising: a chemical composition containing, in mass%, C: 0.01 % to 0.07 %, Si: 0.5 % or less, Mn: 1.0 % to 2.0 %, P: 0.01 % or less, S: 0.0005 % to 0.0050 %, Al: 0.030 % or less, Ni: 0.5 % to 2.0 %, Ti: 0.005 % to 0.030 %, N: 0.0015 % to 0.0065 %, O: 0.0010 % to 0.0050 %, and Ca: 0.0005 % to 0.0060 %, optionally, in mass%, at least one element selected from the group consisting of Cu: 0.05 % to 2.0 %, Cr: 0.05 % to 0.30 %, Mo: 0.05 % to 0.30 %, Nb: 0.005 % to 0.035 %, V: 0.01 % to 0.10 %, W: 0.01 % to 0.50 %, B: 0.0005 % to 0.0020 %, REM: 0.0020 % to 0.0200 %, and Mg: 0.0002 % to 0.0060 %, with a balance being Fe and inevitable impurities; and having Ceq of 0.45 % or less, where Ceq is defined by the following Formula: Ceq % = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5; and having Pcm of 0.22 % or less, where Pcm is defined by the following Formula: Pcm % = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 B, where the brackets in Formula and Formula indicate a content by mass% of an element enclosed in the brackets and have a value of 0 if such an element is not contained; an average effective grain size of 20 μm or less at a mid-thickness part of the steel plate, and porosities having an equivalent circular diameter of 200 μm or more, the number of the porosities per mm 2 being 0.1/mm 2 or less, wherein the methods for measuring the average effective grain size and the number density of porosities are defined in the description. a chemical composition containing, in mass%, C: 0.01 % to 0.07 %, Si: 0.5 % or less, Mn: 1.0 % to 2.0 %, P: 0.01 % or less, S: 0.0005 % to 0.0050 %, Al: 0.030 % or less, Ni: 0.5 % to 2.0 %, Ti: 0.005 % to 0.030 %, N: 0.0015 % to 0.0065 %, O: 0.0010 % to 0.0050 %, and Ca: 0.0005 % to 0.0060 %, C: 0.01 % to 0.07 %, Si: 0.5 % or less, Mn: 1.0 % to 2.0 %, P: 0.01 % or less, S: 0.0005 % to 0.0050 %, Al: 0.030 % or less, Ni: 0.5 % to 2.0 %, Ti: 0.005 % to 0.030 %, N: 0.0015 % to 0.0065 %, O: 0.0010 % to 0.0050 %, and Ca: 0.0005 % to 0.0060 %, optionally, in mass%, at least one element selected from the group consisting of Cu: 0.05 % to 2.0 %, Cr: 0.05 % to 0.30 %, Mo: 0.05 % to 0.30 %, Nb: 0.005 % to 0.035 %, V: 0.01 % to 0.10 %, W: 0.01 % to 0.50 %, B: 0.0005 % to 0.0020 %, REM: 0.0020 % to 0.0200 %, and Mg: 0.0002 % to 0.0060 %, Cu: 0.05 % to 2.0 %, Cr: 0.05 % to 0.30 %, Mo: 0.05 % to 0.30 %, Nb: 0.005 % to 0.035 %, V: 0.01 % to 0.10 %, W: 0.01 % to 0.50 %, B: 0.0005 % to 0.0020 %, REM: 0.0020 % to 0.0200 %, and Mg: 0.0002 % to 0.0060 %, with a balance being Fe and inevitable impurities; and having Ceq of 0.45 % or less, where Ceq is defined by the following Formula: Ceq % = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5; and having Pcm of 0.22 % or less, where Pcm is defined by the following Formula: Pcm % = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 B, where the brackets in Formula and Formula indicate a content by mass% of an element enclosed in the brackets and have a value of 0 if such an element is not contained; an average effective grain size of 20 μm or less at a mid-thickness part of the steel plate, and porosities having an equivalent circular diameter of 200 μm or more, the number of the porosities per mm 2 being 0.1/mm 2 or less, wherein the methods for measuring the average effective grain size and the number density of porosities are defined in the description.