Photoresist composition

A photoresist composition comprising:

• a resin which shows an increase in solubility in an aqueous alkali solution by an action of an acid;
• an acid generator;
• a plasticizer selected from the group consisting of ethylphthalylethylglycolate, tris(2-ethylhexyl) trimellitate and N-butylbenzene sulfonamide;
• and
• a solvent the amount of which is from 40 to 75% by mass of the total amount of the photoresist composition,
• said resin being a resin obtained by reacting a novolak resin, a compound having two or more vinyloxy groups and a resin comprising a structural unit represented by formula (a1-2), a structural unit represented by formula (a2-1), or both of the structural units:
• wherein Ra1′ and Ra2′ each independently represent a hydrogen atom or a C1-C12 hydrocarbon group where a methylene group has optionally been replaced by an oxygen atom or a sulfur atom, Ra3′ represent a C1-C20 hydrocarbon group where a methylene group has optionally been replaced by an oxygen atom or a sulfur atom, or one of Ra1′ and Ra2′ is bonded to Ra3′ to form a C2-C20 divalent hydrocarbon group where a methylene group has optionally been replaced by an oxygen atom or a sulfur atom, and the other represents a hydrogen atom or a C1-C12 hydrocarbon group where a methylene group has optionally been replaced by an oxygen atom or a sulfur atom,
• Ra5 represents a hydrogen atom or a methyl group,
• Ra6 represents a C1-C6 alkyl group or a C1-C6 alkoxy group, and m represents an integer of 0 to 4,
• Ra7 represents a hydrogen atom or a methyl group, and
• Ra10 represents a C1-C6 alkyl group or a C1-C6 alkoxy group, and m′ represents an integer of 0 to 4.