1. A fuel injection controller comprising: a first learning device that has a torque calculating device for calculating an instantaneous torque equivalent as a torque equivalent of an output shaft of an internal combustion engine per unit time by filtering a sensed value of rotation speed of the output shaft and an estimating device for estimating an injection characteristic of an injector of the engine based on the instantaneous torque equivalent; and a second learning device that has an injecting device for injecting equally-divided injection amounts of a required injection amount of fuel for performing feedback control of conforming the sensed value of the rotation speed of the output shaft to a target value and a learning device for learning the injection characteristic of the injector based on the fuel injection amount as of the feedback control, wherein the first learning device learns the injection characteristic of the injector in an operation area of the engine other than an operation area where the second learning device performs the learning.