Printing-fluid containing device including printing-fluid cartridge and adaptor

A printing-fluid containing device configured to be inserted into a cartridge attachment section in an insertion direction includes: a printing-fluid cartridge and an adaptor to which the cartridge is detachably assembled. The cartridge includes: a casing; a supply portion; a detection portion; and an engagement portion. The adaptor includes: an adaptor body; and an electrical interface. The detection portion includes a light accessible portion accessed by light from an outside of the cartridge. The engagement portion is engageable with the cartridge attachment section. The adaptor body has a front wall at its leading end in the insertion direction. The front wall has an opening through which the supply portion extends. The electrical interface is disposed on an outer surface of the adaptor body and electrically connectable to an electric contact of the cartridge attachment section.