Provided are a high-strength steel plate excellent in low -temperature toughness, a high-strength steel pipe made by using the plate as the base material, and processes for the production of both. The steel plate contains Mo: 0.05 to 1.00% and B: 0.0003 to 0.0100% with Ceq of 0.30 to 0.53 and Pcm of 0.10 to 0.20, and has a metal structure which has an area fraction of polygonal ferrite of 20 to 90% with the balance being a hard phase consisting of either bainite or martensite, or both. The steel plate is produced by conducting, successively, strain-introducing rolling at an initiation temperature of (Ar3+60 DEG C) or below, an end temperature of Ar3 or above and a rolling ratio of 1.5 or above, air cooling, and accelerated cooling from a temperature of (Ar3-100 DEG C) to (Ar3-10 DEG C) ata rate of 10 DEG C/s or above.