To provide a method for on-press developing a lithographic printing plate master having an image forming layer containing a hydrophobic preparing precursor without generating scumming even by the continuous on-press developing.

The method for on-press developing the lithographic printing plate master having the image forming layer containing the hydrophobic preparing precursor comprises the steps of aggregating the precursor mixed in a dampening water, and making filtration to remove the precursor by a dampening water purifying means.