PURPOSE: To execute plate crown control without delay by determining roll crown quantity from the difference in the rate of thermal expansion of rolls measured in the central part and edge of a thin strip, substituting the same into the prescribed relational equation and estimating plate crown quantity in real time.

CONSTITUTION: The rates of thermal expansion of a roll 3 are measured in the traverse central part Qt and edge St of the thin strip 1' by precision rangefinders 4, 5 in the process for producing the thin strip 1' by dropping a molten metal 1 onto the surface of the cooling type twin rolls 3, 3'. The roll crown quantity ΔWB is calculated from the difference therebetween. The calculated value ΔWB is substd. into the empirical formula ΔWA=f(ΔWB) expressing the relation between the roll crown quantity ΔWB and the plate crown quantity ΔWA under the same cooling and solidifying conditions and the plate crown quantity ΔWA during the manufacture is estimated in real time. The estimation of the plate crown quantity without delay under the production conditions is thus made possible and the plate crown control with good accuracy is made possible.