We have examined the change in the molecular orientation of the photo-aligned film of polyimide containing azobenzene in the backbone structure (Azo-PI) by a washing treatment, The Azo-PI films were washed in 2-propanol with an ultrasonic cleaner for 5 min and then in pure water for 5 min. No noticeable change was observed in the in-plane molecular orientation of the photo-aligned Azo-PI films. On the other hand, the in-plane molecular orientation of a rubbed polyimide (poly[4,4'-oxydiphenylene-1,2,3,4-cyclobutanetetracarboximide]) film was relaxed by washing. From these results we conclude that the molecular orientation of the photo-aligned Azo-PI films is more stable than that of the rubbed polyimide film.