A locking device for a cover of a magnetic tape cartridge (3) for locking a pivotable cover (4) for covering magnetic tape inlet and outlet ports (16 and 17) of the magnetic tape cartridge, including a projection (70) formed on an arm (67) for pivotally supporting the pivotable cover which is engageable with an engaging claw section (74) of a locking member (73) which further includes an operation section (75) and a resilient section (76). The locking member is pivotally supported on a magnetic tape cartridge case (3) and urged by the biasing force of the resilient section to bring the engaging claw section into engagement with the projection. The operation section of the locking member is located in a position in which it is capable of cooperating with an unlocking pin moved from three directions, to thereby give latitude to the process for loading a magnetic tape cartridge on a magnetic recording and reproducing apparatus.