There is provided an electric power system monitoring apparatus that can estimate dynamic behavior constants using measurement signals and can obtain the accuracy of an estimation result. An apparatus according to the present invention that monitors an electric power system includes a dynamic behavior constant estimation unit to estimate a predetermined dynamic behavior constant from a measurement value obtained from the electric power system, a relative accuracy calculation unit to calculate, from the measurement value, a relative accuracy index that indicates accuracy of a dynamic behavior constant estimation value estimated at the dynamic behavior constant estimation unit, and an estimation error calculation unit to calculate an estimation error from reference dynamic behavior information, the dynamic behavior constant estimation value, and the relative accuracy index. Thus, after the accuracy of a dynamic behavior constant estimation value is grasped, a process using the estimation value can be executed.