A recording medium cartridge comprises an information recording medium, an inner rotor, and a pair of shutter members that are piled and accommodated in a cartridge casing. When the inner rotor is rotated, the opening part of the cartridge casing is opened and closed by the shutter members. On the overlapping surface of the shutter and the inner rotor, a plurality of ribs for reducing the contact area of the shutter and the inner rotor are provided. Further, in side parts of the shutter members, first overlapped parts and second overlapped parts are respectively provided through first step parts and second step parts in the moving directions of the shutter members. When the side parts are overlapped on each other, the first overlapped part of one shutter member and the first overlapped part of the other shutter member are continuously connected together through the first step parts on the upper surface side of the shutter members. In the lower surface side of the shutter members, the second overlapped part of one shutter member and the second overlapped part of the other shutter member are continuously connected together through the second step parts to close the opening part for recording and reproducing information provided in the cartridge casing. The first step parts and the second step parts are arranged in non-overlapped positions. Thus, the contact area of the inner rotor and the shutter on the overlapping surface is reduced to smoothly move the pair of the shutter members. Further, when the pair of the shutter members are closed, the step part exposed on the front surface of the shutter members and the step part exposed on the back surface of the shutter members are reinforced.