A magnetic tape cartridge includes a cartridge casing formed by upper and lower casing halves and a single reel around which a magnetic tape is wound and which is contained in the cartridge casing for rotation. A slide door for opening and closing a tape outlet opening is mounted on the cartridge casing to be linearly slidable along a door passage between a closing position where it closes the tape outlet opening and an opening position where it opens the tape outlet opening. A spring support rod extends in the door passage from the slide door in parallel to the direction of sliding movement of the slide door and a door spring in the form of a coiled spring is fitted on the spring support rod for urging the slide door toward the closing position and holding the same in the closing position.