A magnetic tape cassette which can be smoothly assembled and smoothly operated for a long period of time without any malfunctions in operation. In the magnetic tape cassette, a spring support for holding a hub brake spring and an abutment for provisionally holding an extension of a hub brake spring are pivoted on a lower casing member of a casing in which a magnetic tape is received, and a release element for releasing the provisionally held hub brake spring from the abutment to transfer it to a spring engagement of the hub brake is provided on the upper casing member. Also, the spring engagement is arranged on the other side of the hub brake so as to project therefrom.