To provide a resin which is polymerized and cured by heat or light and capable of forming a cured product excellent in characteristics such as heat resistance, humidity resistance, close adherence to a material to be coated, etc., and its compsn. and a resin so excellent in alkali developing property that it can be used as a photosensitive resin and its compsn.

This modified novolak resin is a modified novolak resin having a structure of arom. rings having a substituted group connected with an alkylene bond, and a part or all of the arom. rings have on their different carbons, a substituting group having an alcoholic hydroxy group and a substituting group having a radical-polymerizable unsaturated double bond.