Abstract: A proposal is made for the use of an alkyl pyruvate or a solvent mixture mainly composed thereof as a solvent in a positive-working photoresist composition comprising an alkali-soluble novolac resin as the film-forming ingredient and a quinone diazide group-containing compound as the photosensitive ingredient. By virtue of the use of the unique solvent, the photoresist composition is advantageous in the outstandingly high stability with little moisture absorption from ambience not to cause precipitation of solid matters in addition to the quite satisfactory properties in other respects as a photoresist composition.