In a magnetic disk cartridge, a slide valve for prevention of erroneous erasure is slidably supported in a cartridge case having a detection hole adapted to be closed by the slide valve when the slide valve is at a predetermined sliding position. A resilient lock arm having projections at the ends is positioned on a side face of the slide valve integrally therewith. A pair of recess portions are positioned integrally with the case for receiving and retaining the projections of the lock arm, so that one of the projections is retained and stopped in one of the recess portions when the slide valve is at the sliding position for closing the detection hole the other projection is retained and stopped in the other recess portion when the slide valve is at the sliding position for opening the detection hole.