To provide carbon steel for a nuclear power plant having more excellent corrosion cracking resistance, to provide a method for producing it and to provide a nuclear power plant.

This carbon steel for a nuclear power plant is the one having a compsn. composed of, by weight, 0.1 to 0.35% C, 0.1 to 0.5% Si, 0.3 to 1.5% Mn, and the balance substantially Fe and having a martensitic structure of ≥90% by area ratio by executing quenching. The nuclear power plant uses the carbon steel for piping and apparatus.