To simplify a sieving process by a method in which materials to be sieved are subjected to a series of pre-treatment including pre-crushing, iron group metal separation, post-crushing, and drying and passed through pneumatic sieving apparatuses arranged in sequence.

Waste is subjected to pre-crushing, first iron group metal separation, post-crushing, second iron group metal separation, drying, nonferrous metal separation and introduced as materials 21 into an inlet of the sieving step 23 of sieving steps 23, 30, 37 arranged in sequence. An air current of an adjustable speed is generated in the sieving steps 23, 30, 37 via cyclones 25, 32, 39 and suction pipes 24, 31, 38 by suction fans 26, 33, 40. In the sieving step 23, fine particles 0-2 mm in particle size are separated upward to be fuel. Materials 28 dropped to the side of the step 23 are introduced into the inlet 29 of the sieving step 30, and middle-sized particles of about 2-10 mm particle size are separated upward. Materials 35 dropped downward are introduced into the inlet 36 of the third sieving step 37.