A photo acid generator represented (1a), wherein R01 and R02 each independently represent a linear monovalent hydrocarbon group having 1 to 20 carbon atoms or a branched or cyclic monovalent hydrocarbon group having 3 to 20 carbon atoms which may be substituted with or interposed by a heteroatom; R03 represents a linear divalent hydrocarbon group having 1 to 30 carbon atoms or a branched or cyclic divalent hydrocarbon group having 3 to 30 carbon atoms which may be substituted with a heteroatom, or interposed by a heteroatom; and R01 and R02 may be mutually bonded to form a ring together with the sulfur atom in the formula. A photo acid generator can give a pattern excellent in resolution and LER and having a rectangular profile in the photolithography using a high energy beam like ArF excimer laser light, EUV, and electron beam as a light source.