A magnetic recording/reproducing device uses a tape cassette which includes a first protective lid facing a magnetized side of a magnetic tape, and a second protective lid facing a non-magnetized side of the tape. When the tape cassette is loaded into the device, the first and second protective lids are moved to form a space defined by them. A tape travel guide (impedance roller) and a mechanism member are partially received in this space. With this arrangement, the projected area of the tape path is reduced without affecting the performance, and the tape cassette and the mechanism can overlap each other to an utmost-limit extent, thereby achieving a compact design of the device.