1. A positive type photoresist composition comprising a mixture of a novolak resin and a photo-sensitive o-quinonediazide compound, wherein said novolak resin is one which is obtained by the addition condensation reaction of a phenol and formaldehyde which is performed at pH 4 to 7 by using as a catalyst an organic acid salt of a divalent metal which is more electropositive than hydrogen, said phenol being at least one compound represented by the formula: ##STR4## wherein R is hydrogen, a methyl group of an ethyl group, said compound being such that the average carbon number in the substituent per one nucleus is 0.5 to 1.5 and the ones with the substituent at the ortho- or para-position with respect to the hydroxyl group account for less than 50 mol %, and said o-quinonediazide compound and said novolak resin being present in a weight ratio of 1:1 to 1:6, said composition in a photoresist use providing a gamma-value of above about 1.6.