PURPOSE: To heat steel product groups of different heating conditions at min. spacings and to heat up the steel products under the assigned heating conditions by conveying the steel products for which the heating conditions vary with the time required for changing the temp. of respective zones in a furnace and the conveying speeds of the steel products at the spacings provided therebetween and changing the furnace temp. of the zones during this time.

CONSTITUTION: The time required for changing the set furnace temp. of the steel product group 24 to the set furnace temp. of the steel product group 25 in respective zones (a), (b)...(f) is calculated from the in-furnace set temp. calculated by a furnace temp. and conveying pattern predicting device 18 from the respective heating conditions of the front and rear steel product groups 24, 25 for which the heating conditions vary and the calculated combustion rate to maintain the set temp. of the steel product group 24. The spacings between the in- furnace steel products of the steel product groups 24, 25 necessary for changing the furnace temp. in the above-mentioned zones are calculated by the change time and the conveying speed of the steel product groups 24, 25 and the charging timings of the steel product group 25 is so determined by a charging timing and charging rate setter 17 as to attain the above- mentioned spacings. The combustion rate of said zones is changed and controlled during the time of the above-mentioned spacings on progression of the transfer of the steel product groups 24, 25 by which the furnace temp. of the respective zones (a), (b)...(f) is controlled and the steel products are continuously heated up.