A cartridge case for a magnetic tape cartridge made up of two mating shell halves are provided with a retaining structure for retaining an in-cartridge memory tag at a given angle in the cartridge case. The retaining structure comprises supporting brace members in one mating shell halve which has an angled surface extending at an elevation angle equal to the given angle and holding brace members in the other mating shell halve which has an angled leading surface extending at a depression angle equal to the elevation angle of the angled surface of the support brace member and a angled trailing surge extending at a depression angle greater than the angled leading surface between which the in-cartridge memory tag is held tightly in position in the cartridge case when the two mating shell halves are temporarily or finally coupled to each other.