A tape cassette including a cassette case; a pair of rotatable tape reels accommodated in the cassette case for winding a magnetic tape therearound; a mouth section formed between and front of the tape reels in the cassette case for receiving a head drum and the like of a recording/reproducing device; a mechanism for allowing pivotal movement of the tape reels to rear corner portions of the cassette, whereby when the tape cassette is loaded into the recording/reproducing device, the tape reels are pivotally moved from an initial position to a retracted position defined at the rear corner portions of the cassette case, thereby expanding the mouth section; and a pair of reel retainers pivotably supported in the cassette case for rotatably supporting the tape reels, respectively, each of the reel retainers having a bottom wall formed with a positioning hole adapted to engage a positioning member provided in the recording/reproducing device. Accordingly, the positions of the tape reels can be stabilized during operation.