 within the range of 0 to 0 m/sec, it is possible to realize a convection heat transfer effective enough to accelerate the cooling rate, while preventing oxidization on the surface of an object-to-be-frozen by keeping a bound water film on the surface of the object-to-be-frozen from evaporating. That is, when the wind velocity is too slow, the heat transfer between the cold air and the object-to-be-frozen will be little, therefore making it difficult to achieve 