A polyimide Langmuir-Blodgett (LB) film with a photoresponsive azobenzene unit was prepared by the precursor method using polyamic acid alkyl amine salts. An in-plane anisotropic photoisomerization of the azobenzene unit was observed when the LB film was irradiated with linearly polarized UV light. Photoisomerization was induced selectively in the plane parallel to the polarization of the UV irradiation in preference to the perpendicular plane. The photoselection by the polarized light was remarkable and little photoisomerization was observed in the perpendicular plane in the polyimide LB film. The in-plane anisotropic photoisomerization resulted in a molecular reorientation of the azobenzene unit in the direction perpendicular to the polarization of the UV irradiation. The resulting dichroism was stable at room temperature for a long time, furthermore, it could be easily erased by irradiation with non-polarized UV light. This property was independent of the polarization angle of the incident UV light and reversible dichroism was photochemically recorded in the LB film in all optical directions.