Abstract: A tape-tension measuring device is made to be interchangeable with a tape cassette in a cassette-type magnetic recording and/or reproducing apparatus for accurately indicating the tension produced in the recording and reproducing modes of such apparatus. The tape-tension measuring device has a casing equivalent to the housing of the tape cassette to be interchanged therewith, and first and second reel assemblies corresponding to the take-up and supply reels of a tape cassette are provided within the casing to receive the take-up and supply reel shafts of the recording and/or reproducing apparatus. The second reel assembly has inner and outer ring portions which are relatively rotatable against the resistance of a spring in response to the tension in a test tape which extends between and is wound on the reel assemblies.