Provided is a polymer that can be used as a base resin for a positive photosensitive resin composition and a negative photosensitive resin composition, wherein the positive photosensitive resin composition and the negative photosensitive resin composition are soluble in an aqueous alkaline solution, can form a fine pattern, can achieve high resolution, and have good mechanical properties even when they are cured at low temperature. Also provided are a positive photosensitive resin composition and a negative photosensitive resin composition using the polymer. The polymer is represented by general formulas (1) and/or (2): wherein T1 and T2 may be the same as, or different from, each other and represent any of —CO— and —SO2—; X1 is a tetravalent organic group; and l is 0 or 1; and X2 is a divalent organic group.