1. An image processing apparatus comprising: a detector that configured to detect a ruling and an angle of a screen from a binary image of a plate, the binary image being obtained as a result of screen processing; a memory configured to store a detection result for the plate for which the ruling and the angle have been detected by the detector; and a converter configured to convert an image obtained as a result of screen processing into a multi-level image by using, for the plate for which the detection result has been stored in the memory, the ruling and the angle in the stored detection result, or by using, for another plate, for which no detection result has been stored in the memory, a ruling and an angle obtained as a result of detection performed by the detector.