An ink jet recorder comprises a carriage for holding an ink jet head, capable of sweeping across a recording medium, an ink cartridge for supplying ink to the head, and a detector for detecting whether the fluid level of the ink inside the ink cartridge is at a prescribed level. At least two detection operations are performed by the detector. If two of those detection operations indicate an ink fluid level short of the prescribed level, ink cartridge replacement is displayed on a display. Erroneous ink level judgments can thus be held to a minimum even when the ink forms waves inside the ink cartridge due to the movement of the carriage in the sweep direction.