Provided are a system and method of detecting an ink use amount of an inkjet printer, which allow detection of an ink use amount of an inkjet printer configured to perform image forming by discharging an inkjet ink on a web-shaped print base material by a single-pass method, and an inkjet printer. A system of detecting an ink use amount of an inkjet printer configured to perform image forming by discharging an inkjet ink on a web-shaped print base material by a single-pass method, includes: an ink tank mass measuring device configured to measure a mass of an ink tank; a comparison operation device configured to perform a comparison operation between a setting reference value of an ink use amount, which is prepared in advance, and a use amount measurement value of the ink tank mass measuring device; and a display device configured to display a warning when a difference between the use amount measurement value and the setting reference value exceeds a range set in advance.