A digital filter including: a separation circuit for generating, from a first complex signal in a frequency domain, a second complex signal being an I component of the first complex signal and a third complex signal being a Q component of the first complex signal, and outputting the second complex signal and the third complex signal; a signal selection circuit for performing switching by selecting, from among a plurality of types of complex signals having different amounts of change in signal amplitude, at least one of a fourth complex signal to be output to a first filter circuit and a fifth complex signal to be output to a second filter circuit, by using the second complex signal and the third complex signal; the first filter circuit for performing filter processing on the fourth complex signal by a first filter coefficient, and outputting a sixth complex signal; the second filter circuit for performing processing on the fifth complex signal by a second filter coefficient, and outputting a seventh complex signal; and a combining circuit for combining the sixth complex signal and the seventh complex signal by adding a real part of the sixth complex signal and a real part of the seventh complex signal and adding an imaginary part of the sixth complex signal and an imaginary part of the seventh complex signal, and generating and outputting an eighth complex signal being a complex signal after combining, wherein the selecting is selecting from a first combination of the second complex signal to be output to the first filter circuit and the third complex signal to be output to the second filter circuit, and a second combination of the first complex signal to be output to the first filter circuit and a signal indicating a value zero to be output to the second filter circuit. a separation circuit for generating, from a first complex signal in a frequency domain, a second complex signal being an I component of the first complex signal and a third complex signal being a Q component of the first complex signal, and outputting the second complex signal and the third complex signal; a signal selection circuit for performing switching by selecting, from among a plurality of types of complex signals having different amounts of change in signal amplitude, at least one of a fourth complex signal to be output to a first filter circuit and a fifth complex signal to be output to a second filter circuit, by using the second complex signal and the third complex signal; the first filter circuit for performing filter processing on the fourth complex signal by a first filter coefficient, and outputting a sixth complex signal; the second filter circuit for performing processing on the fifth complex signal by a second filter coefficient, and outputting a seventh complex signal; and a combining circuit for combining the sixth complex signal and the seventh complex signal by adding a real part of the sixth complex signal and a real part of the seventh complex signal and adding an imaginary part of the sixth complex signal and an imaginary part of the seventh complex signal, and generating and outputting an eighth complex signal being a complex signal after combining, wherein the selecting is selecting from a first combination of the second complex signal to be output to the first filter circuit and the third complex signal to be output to the second filter circuit, and a second combination of the first complex signal to be output to the first filter circuit and a signal indicating a value zero to be output to the second filter circuit.