A tape cassette driving system of a video tape recorder for driving the reels of the tape cassette comprises a main chassis (500) for supporting a standard size tape cassette and a sub-chassis (609) for supporting a compact size tape cassette. The main chassis carries a reel disk (607) for driving a take up reel of the standard size tape cassette and the sub-chassis carries a drive gear (612) adapted for engagement with the reel disk on the main chassis and further for engagement with teeth formed on the periphery of the take up reel of the compact size tape cassette. The sub-chassis is connected to the main chassis by a plurality of swing arms (610a - 610d) such that the sub-chassis is movable up and down relative to the main chassis. As a result of the connection by the swing arms, the sub-chassis is moved along a circular path when it is moved up and down and the engagement of the drive gear with a toothed part (607a) of the reel disk can be achieved smoothly. Further, the reel disk comprises a braking mechanism (647 - 649) for preventing the reel disk from rotating when the reel disk is moved up and down. The braking mechanism is controlled by a mechanism (603) which also controls the movement of the sub-chassis up and down.
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