The present invention: enables the resistance coefficient of a nozzle to be decreased and a cooling device assembly to be downsized in a cooling device requiring a higher cooling rate; in particular, the invention is a cooling device for a steel strip, said cooling device being equipped with a plurality of nozzles protruding from the surfaces of a cooling chamber so as to keep the distance between the tips of said nozzles and the surfaces of said steel strip in the range from 30 to 100 mm and cooling said traveling steel strip by ejecting a cooling medium through said nozzles, characterized in that D/d of said blowing nozzles satisfies the expression 1.5 ( D/d ( 3.0 wherein d is the inner diameter at the tip of a nozzle (steel strip side) and D the inner diameter at the foot of a nozzle (cooling chamber side).
