1. An image data producing apparatus for producing proofing halftone dots image data representative of a proof image in which an image obtained by a printing system is simulated, said printing system printing the image in accordance with printing halftone dots image data subjected to halftone dots processing for multi-tone level image data, said image data producing apparatus comprising: a halftone dots pattern data storage section for storing a plurality of sorts of halftone dots pattern data representative of a plurality of sorts of halftone dots patterns; an image data obtaining section for obtaining the multi-tone level image data; a halftone dots pattern selecting section for selecting a single halftone dots pattern according to an operation from among the plurality of sorts of halftone dots patterns represented by the plurality of sorts of halftone dots pattern data stored in said halftone dots pattern data storage section; a halftone dots image data producing section for producing the proofing halftone dots image data by means of subjecting the multi-tone level image data obtained by said image data obtaining section to the halftone dots processing based on the halftone dots pattern data representative of the halftone dots pattern selected by said halftone dots pattern selecting section; and an image data output section for outputting the proofing halftone dots image data produced by said halftone dots image data producing section.