Photosensitive composition and planographic printing plate precursor using same

A photosensitive composition comprising components (a) and (b) as follows, and becoming soluble in an aqueous alkaline solution when irradiated with an infrared laser:

• (a) an infrared-ray absorbing agent represented by formula (I) as follows;
• and
• (b) a polymeric compound insoluble in water but soluble in an aqueous alkaline solution
• wherein R1 through R4 each independently represents a hydrogen atom, alkyl group or aryl group;
• R5 and R6 each independently represents an alkyl group, substituted oxy group or halogen atom;
• n and m each independently represents an integer from 0 through 4;
• R1 and R2, or R3 and R4 may combine with each other to form a ring, and R1 and/or R2 may combine with R5, or R3 and/or R4 may combine with R6, to form a ring;
• a plurality of R5 or a plurality of R6 may combine together to form a ring;
• Z1 and Z2 each independently represents a hydrogen atom, alkyl group or aryl group, at least one of Z1 and Z2 representing a hydrogen atom or an alkyl group;
• Q represents a trimethyne group or pentamethyne group, each of which may have a substituent group, and is able to form a cyclic structure together with a divalent organic group;
• and X− represents a counter anion.