To provide a photosensitive polymer, a phase difference film using the photosensitive polymer, and an optical film using the phase difference film. A solution is a photosensitive polymer having a constitutional unit represented by formula (1): wherein, in formula (1), for example, R1 is hydrogen or methyl; a is 2 or 3; p is an integer from 1 to 12; X1 is —O—, —COO— or —OCO—; b is an integer from 0 to 3; A1 is a divalent group selected from 1,4-phenylene or naphthalene-2,6-diyl, and in the divalent group, at least one of hydrogen may be replaced by fluorine or chlorine; Z1 is a single bond, —COO—, —CH═CH—COO—, —CH2CH2—COO—, —CH2O— or —CONH—; W1 and W2 are hydrogen, fluorine or 1-5C alkyl or alkoxy; Y1 is —O—; and R2 is 1-20C alkyl, and at least one of —CH2— in the alkyl may be replaced by —O—.