the light receiving element 30 b further in the state where the ink cartridges 32 a 32 b have been mounted on the cartridge holder 31 since the reflectors 40 a 40 b are covered with the recesses 41 a 41 b the ink will not adhere to the reflectors 40 a 40 b even though the ink is splashed during the printing operation as a result it is possible to prevent an erroneous detection caused by ink adhering to the reflectors 40 a 40 b fig1 is a sectional view of the ink supply section taken along a line x x in fig7 showing a state where the ink cartridge is plenarily mounted and fig1 is a sectional view of the ink supply section taken along a line y y in fig7 showing a state where the ink cartridge has been provisionally mounted as shown in these drawings there are provided in a rear part of the cartridge holder 31 two lock plates 43 a 43 b in an upright manner the lock plates 43 a 43 b are elastic plate members formed with v shaped locking portions 44 a 44 b in their upper end portions when the ink cartridges 32 a 32 b have been mounted on the cartridge holder 31 from above convex portions 45 a 45 b projected from back faces of the ink cartridges 32 a 32 b lie on the.