A mineral material processing plant includes a frame and a screen which is supported on the frame of the processing plant, and a conveyor which is fixed in connection with the screen for transporting material to be processed by the screen. The support of the conveyor relative to the screen includes a four-bar linkage mechanism, and pivot points of the four-bar linkage mechanism are arranged in support arms which are fixed pivotably in connection with the screen, and the conveyor is arranged to be moved further from the screen and closer to the screen by the movement of the four-bar linkage. A method for operating a processing plant is also disclosed.