PURPOSE: To reduce the cost of equipment by beforehand finding the optimum cooling water rate to the setting casting velocity by using heat transfer model, correcting hystrical casting velocity by present casting velocity and correspond ing it to the setting casting velocity.

CONSTITUTION: Whole length of a cast slab 1 drawn from a mold 2 is divided into plural zones, and to each control zone, the aimed surface temp. TA, TB, TC corresponding to operational condition of casting velocity, etc., is set. Next, the optimum water rate for each control zone is calculated by thermal calcula tion giving the standard cooling water rate, and the relational curve between the casting velocity and the water rate, is found based on plural casting veloci ties and the optimum cooling water rate for each zone is set by off-line. Succes sively, the hystrical casting velocity considering variation of the velocity to time in the past, is corrected by the casting velocity at the present time, to decide the optimum cooling water rate. As a large-scaled computor is not neces sary because of decrease of the calculation treating quantity, the cost of equip ment is remarkably reduced.