A negative planographic printing plate precursor for heat-mode exposure systems, which has, on a support, a photosensitive layer containing: (A) a light-to-heat conversion agent; (B) a polymerizable unsaturated group-having compound; and (C) an onium salt having a counter anion with a valency of at least 2. The precursor is capable of being exposed with an IR laser for image formation thereon. The onium salt may be, for example, a diazonium salt, iodonium salt or sulfonium salt. The counter anion has at least two anionic sites which may be the same or different, and the anionic structure is preferably divalent to hexavalent.