Abstract: According to the present invention there are provided a method and an apparatus capable of disintegrating used paper into long pulp fibers without tearing it mechanically. In the apparatus, a wall surface having an undulating section is formed on an inner side of a gas flow path, with a current of air having a velocity component parallel to the wall surface and a velocity component perpendicular to and striking against the wall surface is allowed to pass through the gas flow path. Finely cut used paper pieces are allowed to pass through the gas flow path while being carried by the air current. The parallel velocity component of the air current is set at 40 m or more. An impeller is mounted within a circular casing body having an inlet port and an outlet port, and undulating teeth are formed on the inner peripheral surface of the circular casing body. The used paper, which is sucked in from the above inlet port together with the air current, is cut beforehand into pieces.