A high-strength steel plate excellent in deformability and low-temperature toughness, comprising: in mass%, C: 0.010 to 0.080%; Si: 0.01 to 0.50%; Mn: 1.2 to 2.8%; S: 0.0001 to 0.0050%; Ti: 0.003 to 0.030%; B: 0.0003 to 0.005%; N: 0.0010 to 0.008%; O: 0.0001 to 0.0080%; one or more of Cr, Cu, and Ni; P: limited to 0.050% or less; Al: limited to 0.020% or less; Mo: limited to 0.03% or less; a Ceq obtained by (Expression 1) below of 0.30 to 0.53; and a Pcm obtained by (Expression 2) below of 0.10 to 0.20; and a balance being composed of iron and inevitable impurities; wherein a metal structure contains 27 to 90% in area ratio of polygonal ferrite and a hard phase composed of one or both of bainite and martensite as its balance. Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo / 5 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + 5 ⁢ B In (Expression 1) and (Expression 2) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective contents are 0%, wherein the high-strength steel plate excellent in deformability and low-temperature toughness optionally further comprises: one or two or more of, in mass%, W: 0.01 to 0.50%; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; REM: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%; wherein the Ceq is obtained by (Expression 1') below in place of (Expression 1) above, and the Pcm is obtained by (Expression 2') below in place of (Expression 2). Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 ⁢ B In (Expression 1') and (Expression 2') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective contents are 0%. V is calculated as 0 when the content is 0% and the content is less than 0.010 mass%. in mass%, C: 0.010 to 0.080%; Si: 0.01 to 0.50%; Mn: 1.2 to 2.8%; S: 0.0001 to 0.0050%; Ti: 0.003 to 0.030%; B: 0.0003 to 0.005%; N: 0.0010 to 0.008%; O: 0.0001 to 0.0080%; one or more of Cr, Cu, and Ni; P: limited to 0.050% or less; Al: limited to 0.020% or less; Mo: limited to 0.03% or less; a Ceq obtained by (Expression 1) below of 0.30 to 0.53; and a Pcm obtained by (Expression 2) below of 0.10 to 0.20; and a balance being composed of iron and inevitable impurities; wherein a metal structure contains 27 to 90% in area ratio of polygonal ferrite and a hard phase composed of one or both of bainite and martensite as its balance. Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo / 5 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + 5 ⁢ B In (Expression 1) and (Expression 2) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective contents are 0%, wherein the high-strength steel plate excellent in deformability and low-temperature toughness optionally further comprises: one or two or more of, in mass%, W: 0.01 to 0.50%; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; REM: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%; wherein the Ceq is obtained by (Expression 1') below in place of (Expression 1) above, and the Pcm is obtained by (Expression 2') below in place of (Expression 2). Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 ⁢ B one or two or more of, in mass%, W: 0.01 to 0.50%; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; REM: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%; wherein the Ceq is obtained by (Expression 1') below in place of (Expression 1) above, and the Pcm is obtained by (Expression 2') below in place of (Expression 2). Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 ⁢ B the Ceq is obtained by (Expression 1') below in place of (Expression 1) above, and the Pcm is obtained by (Expression 2') below in place of (Expression 2). Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 ⁢ B