PURPOSE: To obtain the radiation sensitive positive composition high in sensitivity by using a phenol resin binder soluble not in water but in an alkali and prepared by substituting phenolic hydroxyl groups with acetal groups.

CONSTITUTION: The radiation sensitive composition comprises (a) the binder or its mixture soluble not in water but in an alkali, and (b) a compound to be allowed to form a strong acid by irradiation with radiation. The binder (a) is the phenol resin having 20-70% of its hydroxyl groups substituted by a group represented by the formula in which each of R1 and R2 is an alkyl group; R3 is an H atom or an alkyl group; and each of R1 and R2 may combine with each other together with -(CH2)m- to form ring; and (m) is an integer of 3-6. It is preferred for the phenol resin (a) to have an average molecular weight of 300-20000g/mol, especially, 300-2000g/mol.