An ink cartridge has a sensor to detect ink. A printer's control device transmits a detection command and specified detection condition to the ink cartridge by radio communication. In response to input of the detection command into the ink cartridge, a sensor controller actuates and vibrates the sensor under the specified detection condition. The sensor is attached to a resonance chamber disposed in an ink chamber. The frequency of vibration of the sensor is regulated by a resonance frequency of the resonance chamber. The resonance frequency is varied by ink in the resonance chamber. Detection of the resonance frequency specifies if ink is in the resonance chamber and the remaining quantity of ink in the ink cartridge. The printer's control device receives the detection result together with the detection condition from the ink cartridge, and cheeks if detection was carried out under the specified detection condition, validate the detection result.