An active noise reduction device includes a standard signal generator, adaptive filters, a control sound emitter, and an error signal detector. The standard signal generator generates a standard signal including harmonics of a fundamental frequency correlated with the control target sound. The adaptive filters each generates corresponding one of harmonic components of a control signal based on the standard signal. The adaptive filters each is for corresponding one of the harmonics. The control sound emitter emits a control sound based on the control signal. The error signal detector collects residual noise left over after interference between the control target sound and the control sound, and detects an error signal based on the residual noise. Each of the adaptive filters includes a step size determiner. The step size determiner sets the step size parameter based on a frequency variation in the corresponding one of the harmonic components of the standard signal.