PURPOSE: The flour of grain as wheat or rice flour is brought into contact with a low-temperature medium as liquefied natural gas or liquified petroleum gas and rapidly frozen to expand the water in the grain and make the tissue porous and the weight light, thus producing said grain flour with full and smooth surface.

CONSTITUTION: A grain flour as rice or wheat flour is rapidly frozen by the low temperature of liquefied natural gas or petroleum gas and the tissue is made porous and light. In an example, a liquefied natural gas is charged in the outer space 3 of double-walled vessel 1 and the grain flour in the inner space is precooled. Then, the liquefied gas 4 is sent into the outer space 9 of zig-zag double-walled pipe 7, circulation pipe 12 and returned to tank 5. Machine 13 is driven to operate screw feeder 8 and pass the grain flour through the inner part 10 of the pipe and the flour is pneumatically sent to cyclon 11. The flour is frozen to -50W-60°C in a few minutes. Plates 16 help the efficient cooling of the flour and fragile grains collide with plates 16 and inner surface of pipe 7 into fine particles.