PURPOSE: To increase strength while maintaining high toughness by heating a steel slab with specific composition at a specific high temp. and performing solution heat treatment for the precipitate of Nb.

CONSTITUTION: The steel has a composition which contains 0.10% C and 0.015-0.06% Nb and where PCM is regulated to ≤0.20%. The slab of the steel is subjected to low temp. heating at ≤1130°C and successively to controlled rolling or is subjected, besides controlled rolling, to controlled cooling, by which a non-thermal refining high toughness steel having high tensile strength can be produced. At this time, the slab is previously heated at a temp. as high as ≥1200°C to undergo the solution heat treatment for Nb precipitate. By this method, a strength increasing means capable of maintaining the chemical components of the non-thermal refining high toughness steel at low level without causing deterioration in characteristics due to rolling conditions and cooling conditions can be provided.