Positive photosensitive composition and method of forming pattern using the same

A positive photosensitive composition comprising:

• a resin (A) whose dissolution rate in an alkaline developing solution increases by the action of an acid, the resin (A) containing an acid decomposable repeating unit represented by a general formula (I) and an acid nondecomposable repeating unit represented by a general formula (II);
• and
• a compound (B) capable of generating an acid upon irradiation with one of active rays and radiations:
• wherein
• Xa1 represents one of a hydrogen atom, an alkyl group, a cyano group and a halogen atom,
• A1 represents one of a single bond and a divalent connecting group,
• ALG represents an acid leaving hydrocarbon group and has a polycyclic hydrocarbon structure,
• Xa2 represents one of a hydrogen atom, an alkyl group, a cyano group and a halogen atom,
• A2 represents one of a single bond and a divalent connecting group, and
• ACG represents an acid nonleaving hydrocarbon group and does not have a polycyclic hydrocarbon structure.