A steel plate for a high-strength line pipe having excellent low temperature toughness, comprising, in percent by mass: C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: more than 1.2% to 2.0%, P: exceeding 0% and 0.010% or less, S: exceeding 0% and 0.01% or less, Al: 0.010 to 0.080%, Nb: 0.002 to 0.060%, Ti: 0.003 to 0.030%, Ca: 0.0003 to 0.0060%, N: 0.0010 to 0.010%, REM: 0.0001 to 0.0300%, and Zr: 0.0001 to 0.0200%, optionally further comprising one or two or more elements selected from the group consisting of, in percent by mass: Cu: exceeding 0% and 1.50% or less, Ni: exceeding 0% and 1.50% or less, Cr: exceeding 0% and 1.50% or less, Mo: exceeding 0% and 1.50% or less, and V: exceeding 0% and 0.1% or less, with the balance being iron and inevitable impurities, wherein an average grain size of the steel plate is 10 μm or less in a t/4 position where t is a thickness of the steel plate, and a separation index SI measured on a fracture surface of a 2 mm V notched Charpy specimen of the steel plate in conformity with ASTM-A370 standard at a specified temperature exceeds 0 mm/mm 2 and is 0.30 mm/mm 2 or less, wherein the steel plate does not have the following composition in percent by mass: C: 0.07%, Si: 0.32%, Mn: 1.44%, P: 0.006%, S: 0.0009%, Al: 0.025%, Ca: 0.0036%, N: 0.0047%, O: 0.0021%, Ti: 0.011%, Cu: 0.15%, Ni: 0.23%, Cr: 0.26%, Mo: 0.09%, Nb: 0.010%, REM: 0.0021%, and Zr: 0.0011%, with the balance being iron and inevitable impurities. C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: more than 1.2% to 2.0%, P: exceeding 0% and 0.010% or less, S: exceeding 0% and 0.01% or less, Al: 0.010 to 0.080%, Nb: 0.002 to 0.060%, Ti: 0.003 to 0.030%, Ca: 0.0003 to 0.0060%, N: 0.0010 to 0.010%, REM: 0.0001 to 0.0300%, and Zr: 0.0001 to 0.0200%, optionally further comprising one or two or more elements selected from the group consisting of, in percent by mass: Cu: exceeding 0% and 1.50% or less, Ni: exceeding 0% and 1.50% or less, Cr: exceeding 0% and 1.50% or less, Mo: exceeding 0% and 1.50% or less, and V: exceeding 0% and 0.1% or less, with the balance being iron and inevitable impurities, wherein an average grain size of the steel plate is 10 μm or less in a t/4 position where t is a thickness of the steel plate, and a separation index SI measured on a fracture surface of a 2 mm V notched Charpy specimen of the steel plate in conformity with ASTM-A370 standard at a specified temperature exceeds 0 mm/mm 2 and is 0.30 mm/mm 2 or less, wherein the steel plate does not have the following composition in percent by mass: C: 0.07%, Si: 0.32%, Mn: 1.44%, P: 0.006%, S: 0.0009%, Al: 0.025%, Ca: 0.0036%, N: 0.0047%, O: 0.0021%, Ti: 0.011%, Cu: 0.15%, Ni: 0.23%, Cr: 0.26%, Mo: 0.09%, Nb: 0.010%, REM: 0.0021%, and Zr: 0.0011%, with the balance being iron and inevitable impurities. Cu: exceeding 0% and 1.50% or less, Ni: exceeding 0% and 1.50% or less, Cr: exceeding 0% and 1.50% or less, Mo: exceeding 0% and 1.50% or less, and V: exceeding 0% and 0.1% or less, with the balance being iron and inevitable impurities, wherein an average grain size of the steel plate is 10 μm or less in a t/4 position where t is a thickness of the steel plate, and a separation index SI measured on a fracture surface of a 2 mm V notched Charpy specimen of the steel plate in conformity with ASTM-A370 standard at a specified temperature exceeds 0 mm/mm 2 and is 0.30 mm/mm 2 or less, wherein the steel plate does not have the following composition in percent by mass: C: 0.07%, Si: 0.32%, Mn: 1.44%, P: 0.006%, S: 0.0009%, Al: 0.025%, Ca: 0.0036%, N: 0.0047%, O: 0.0021%, Ti: 0.011%, Cu: 0.15%, Ni: 0.23%, Cr: 0.26%, Mo: 0.09%, Nb: 0.010%, REM: 0.0021%, and Zr: 0.0011%, with the balance being iron and inevitable impurities.