A printing-fluid containing device configured to be inserted into a cartridge attachment section in an insertion direction to be detachably attached to the cartridge attachment section includes a printing-fluid cartridge and an adaptor. The cartridge includes: a casing; a supply portion; and a detection portion. The adaptor, to which the printing-fluid cartridge is configured to be detachably assembled, includes: an adaptor body into which the printing-fluid cartridge is insertable; an electrical interface; and an engagement portion. The adaptor body has a leading end and a trailing end in the insertion direction. The adaptor body has a front wall at the leading end, has an opening through which the supply portion extends, and further has an outer surface. The electrical interface is disposed on the outer surface. The engagement portion is configured to engage with the cartridge attachment section.