A photoresist composition comprising an acid generator and a resin comprising a structural unit derived from a monomer represented by the formula (1): wherein R1 represents a hydrogen atom etc., R2 represents a hydrogen atom or a C1-C4 alkyl group, A1 represents a single bond etc., and B1 represents a group represented by the formula (1a): wherein R3, R4 and R5 each independently represents a C1-C16 aliphatic hydrocarbon group and R4 and R5 can be bonded each other to form a C3-C18 ring together with the carbon atom to which R4 and R5 are bonded, a C4-C20 saturated cyclic group having a lactone structure or a C5-C20 saturated cyclic hydrocarbon group having at least one hydroxyl group.