The invention provides an ink ribbon cartridge which ensures proper, reliable mounting of an ink ribbon. The ink ribbon cartridge of the invention includes a pair of spools, an ink ribbon including a band-like sheet wound around the pair of spools and provided with a layer of ink formed on one surface thereof, four spindles which have disk-shaped sections at one end and can be placed, for a specific length in both ends of the pair of spools, and a cover covering the pair of spools around which the sheet is wound and having four round holes in which the disk-shaped sections of each of the spindles are loosely fitted. In one of the four ends of the pair of spools, only the other end of a specific one of the four spindles can be inserted for a specific length, and one of the four round holes formed in the cover is formed such that only the disk-shaped section on one side of the specific spindle can be loosely fitted.