PURPOSE: To provide a film unit with a lens which is prevented from inadvertent being welded at a point exclusive of an engaging part by forming the film unit in such a manner that only the engaging part of a main body part and a rear cover is rapidly and surely welded.

CONSTITUTION: The engaging part 17 having a projecting shape is formed at the main body part 2 and the engaging part 18 having a recessed part at the rear cover 4, respectively. Energy directors 19a to 19c having sharp angled tips are formed on three surfaces 18a to 18c of the engaging part 18 facing the engaging part 17 and varying in height only at the one surface. The rear surface side of the rear cover 4 contains an ultrasonic vibrator element 21 and an ultrasonic welding unit 22 which moves vertically is held pressed thereto. An anvil 23 is held pressed to the rear surface of the engaging part 17 of the main body part 2 on the opposite side.