An ink ribbon cartridge includes spools each including a ratchet gear in one end portion. A cartridge body includes integrally formed resilient engagement pieces each including a ratchet portion that engages and disengages the ratchet gear of the respective spool and a compressed portion that is compressed by a compressing portion of a thermal transfer printer apparatus. When the ink ribbon cartridge is installed in the thermal transfer printer apparatus, the resilient engagement pieces behave such that the compressed portions are compressed by the compressing portions of the thermal transfer printer apparatus. Thereby, engagement between the ratchet gears of the respective spools and the ratchet portions is released, and the spools become rotatable in the cartridge body.