To prevent the liquid leakage of the electrolyte from a power source battery without drastically changing the structure of the conventional film unit with a lens.

A battery case comprises a battery case body 54 and a cap 55. The battery case is formed to the dimensions approximately the same as those of a battery of a size AA type. A cap 55 is formed of elastomers. A positive pole contact piece 52 is mounted at the battery case body 54. A terminal 53b of a negative pole contact piece 53 is impaled into the cap 55. A power source battery 50 of a size AAA is housed into the battery case body 54 and the cap 55 is mounted at the battery case body 54. A terminal 52b of a position pole terminal 52 is impaled into the cap 55 and the cap 55 water-tightly covers the battery case body 54. The terminals 52b and 53b are mounted at a positive pole side electric contact piece 60 and a negative pole side electric contact piece 61. The positive pole side electric contact piece 60 and the negative pole side electric contact piece 61 are mounted at the circuit board of a stroboscopic unit. The battery case has the dimensions approximately the same as those of the battery of the size AA type and can therefore be mounted at the conventional film unit with the lens.