PURPOSE: To produce a spheroidal graphite cast iron having excellent mechanical properties by subjecting the spheroidal graphite cast iron contg. specific ratios of Mn and Mo successively to high-temp. austenitization, quick cooling, long- period bainite formation and subzero treatment under specific conditions.

CONSTITUTION: The spheroidal graphite cast iron contg. ≥0.8wt% Mn or Mo alone or both thereof is subjected to the high-temp. austenitization by heating the same for 15minW5hr in a 925W1,050°C temp. range. After such cast iron is cooled down to 200W450°C at the speed at which the pearlite transformation is substantially not induced, the cast iron is converted to bainite by the constant temp. holding for 45minW20hr at 200W450°C as it is or by heating up and cooling down the cast iron. The cast iron is further subjected to the subzero treatment in the state of the constant temp. holding or from the cooled state thereafter. The structure contg. ≥35% residual austenite, 40W80% sum of the residual austenite and bainite and 20W60% martensite in the volume ratio of the cast iron base is thereby formed and the spheroidal graphite cast iron having high strength and adequate toughness is obtd.