To provide a novolac type phenol resin and a production method of a novolac type phenol resin, from which a photoresist composition having high resolution, high sensitivity, high film remaining rate and high heat resistance in a good balance can be produced, and to provide a photoresist composition.

The novolac type phenol resin is obtained by a condensation polymerization reaction of a phenol component (a) and an aldehyde component (b). The phenol component (a) essentially comprises m-cresol and/or p-cresol; and the aldehyde component (b) essentially comprises alkoxy benzaldehydes (b1) and formaldehyde (b2). A production method of the novolac type phenol resin is provided, and a photoresist composition including the above resin is provided.