To reduce the variation in water content of a spray dried product by using an apparatus comprising a means for spray drying a detergent slurry with rotary flow nozzles, a control means for controlling the spray pressure or spray rate of the slurry, and a control means for controlling at least either the spray pressure or the spray rate.

A detergent slurry 2 in a slurry tank 1 is pressurized with a pump 7 driven by a motor 6. The pressure generated by the pressurizing, i.e., the spray pressure, is measured with a pressure gage 10 and is fed back to a control apparatus 11 to control the driving of the motor 6, controlling the spray pressure. The amt. of the slurry supplied is controlled by the number of revolutions of the motor 6, the number of the rotary flow nozzles 5 for spraying, and the sizes of the nozzles. The amt. of the slurry supplied is measured with a pressure gage 9 and is fed back to the control apparatus 11 to control the number of the spray nozzles. Thus, the amt. of the slurry supplied, i.e., the amt. of it sprayed, is controlled, and the product of spray pressure and spray amt. is controlled to be in a specified range.