can satisfactorily be achieved even by using the fixed semi cylindrical tape guide fig6 to 9 show a structure of a main portion of the lower half 2 of a tape cassette according to an exemplary embodiment of the invention fig1 to 13 show the structure in the state in which a cylindrical tape guide 5 is mounted on the other hand fig1 to 17 show the same structure having however a semi cylindrical tape guide mounted therein referring to fig6 to 9 a tape guide supporting structure provided on the inner surface of the lower half of the cassette casing includes a supporting pin 4 which can engage with the bore of the cylindrical tape guide 5 and has a diameter same as or only slightly smaller than the inner diameter of the tape guide 5 and a platform or supporting surface 10 formed around the base portion of the supporting pin 4 for supporting the bottom end of the tape guide by virtue of this structure the cylindrical tape guide can fixedly be supported further formed in the tape supporting surface surrounding partially the base portion of the pin 4 is a semi circular groove 12 which has a bottom surface at a lower level than the supporting surface top surface of the platform 10 as can be seen in fig6 and 7 the inner wall of this groove 1 is provided with an arcuate supporting rib 15 having a radius of curvature corresponding to the inner diameter of the semi cylindrical tape guide while the outer.