To provide a cassette, in which width of an outer plate is matched to width of one surface of a case of an image forming device to improve appearance of the image forming device without greatly increasing production cost, and a recording medium carrying device and an image forming device using the cassette.

A left end of a front surface part 25a composing a center of the outer plate of the cassette is set as a thin part 201 formed as a cut in the form of a step from the front surface side. In a right end surface of a left side extended part 25c, a thin wall part 202 is protruded along the front surface in similar size to the cut in the front surface part 25a. The front surface part 25a and the left side extended part 25c are combined with each other to put the thin wall parts 201 and 202 to each other, and screws 203 are driven into screw holes 201a and 202a to form the outer plate in such size that the front surface part 25a and the left side extended part 25c are continuous. By changing the left side extended part 25c, the width of the outer plate can be varied to apply to model change.