To suppress the amt. of NOx to be formed in the case steel products for hot rolling are heated by a combustion heater.

The continuous heater is a multizone type walking beam type heating furnace for continuously heating the continuous hot rolled steel products is ≤1.5 m in the height of at least one zone among the upper zones or lower zones having at least one combustion device. In addition, the opening angle of at least one combustion burner installed in the zone of ≤1.5 m in the height of the zone with respect to the height direction of the heating furnace of a straight line connecting the center of a combustion supporting gas supply hole 14 and the center of a combustion gas supply hole 15 is ≤25°. Even if the height of the zone decreases, the heating of a slab may be executed by suppressing the amt. of the NOx to be formed.