Abstract: One end portion of a wire net is fixedly secured to a fixed holder on a base frame of a sieving machine provided with a vibration device, the other end portion thereof is fixedly secured to a movable holder that is provided on the base frame so as to be freely movable, this movable holder is biased in the direction of separating the movable holder from the fixed holder by means of a spring provided on the base frame, and also, on the base frame is provided a hydraulic device which moves the movable holder in the direction of approaching to the fixed holder against the resilient force of the spring. During operation of this sieving machine, in orer to clear blocking of the meshes of the wire net, the hydraulic device is actuated only when the particles of raw material which caused the blocking are shaken off by relaxing the wire net and broadening the distance between adjacent wires in the wire net.