An ink container detachably mountable to an ink jet recording apparatus which includes a cartridge provided with a recording head and capable of detachably carrying the ink container, and includes a mechanical switch for detecting mounting of the cartridge by its displacement, the container including a bottom side which is provided with an ink supply port for supplying the ink from an inside of the container to the recording head and which takes a bottom position in use; a substantially vertical side having an engaging portion for mounting the ink container to the cartridge; a stepped portion, in the bottom side, forming a recess having an end which is open at the vertical side; and a projected abutment portion, provided adjacent the vertical side in the recess, for displacing the mechanical switch.