A miniturized video magnetic tape cassette of the type in which a magnetic tape wound on a pair of hubs in the hollow interior of a cassette casing is guided to run across a front opening formed in the casing along a rotatable front cover and in which the tape can be pulled out of the front opening in the cassette. The tape cassette includes locking mechanism having a supporting shaft fixed to a bottom wall of the casing and a hub rotatably mounted on the supporting shaft. An L-shaped locking lever extends from the hub and a first stop extends from the hub in a direction opposite that of the locking lever. A coil spring has a coil portion disposed around an upper portion of a hub in which a slit is formed. A first end of the coil spring is inserted into the slit while the second end abuts against an inner surface of a side wall of the casing.