A controller and method of a reducing agent injector capable of accurately providing a required amount of ammonia to a reduction catalyst.A controller for controlling energization of an electromagnetic solenoid, including: a target injection amount calculator for calculating a target injection amount of the reducing agent; a first current output signal calculator for calculating the duration of providing a first current output signal to be provided to the electromagnetic solenoid at the start of injection and calculating the DUTY ratio of the first current output signal; a second current output signal calculator for calculating the DUTY ratio of providing a second current output signal after the first current is provided; a temperature calculator for estimating a temperature within the reducing agent injector; and an output signal corrector for correcting at least one of the first current output signal and the second current output signal based on the temperature estimated.