The injection error monitor is now being developed for an easy tuning of the main ring beam injection at the KEK 12 GeV proton synchrotron. It is expected to perform the automatic injection tuning for the main ring by using this monitor system. The beam trajectory and the tuning ability of the injection equipment on the horizontal phase plane are obtained and confirmed by a test bench system. The injection error monitor has been proved to be efficient for the beam injection tuning. © 1994 American Institute of Physics