PURPOSE: To enable an adapter for correcting a diameter which holds a small- diameter cartridge to be held under a state that it is housed in the cartridge chamber of a camera when it is always housed by providing a fixing means on the adapter.

CONSTITUTION: When the adapter 20 is fixed in the cartridge chamber 310, a cartridge housing part 220 is inserted in the chamber 310 of the camera 30 by aligning the recessed part 261 of the housing part 220 with the position of a fork for rewinding 315 in the chamber 310. Then, a screw for fitting 40 is inserted through a screw inserting hole 222 from the inside circumferential surface 250 side of the adapter 20 and screwed to the screw hole 317 of the chamber 310. Thus, the adapter 20 is fixed under the state it is housed in the chamber 310 and the adpapter is not removed from the chamber 310 every time that the small-diameter cartridge 10 is loaded or unloaded with respect to the adpater 20.