A mixing device of a first signal and a second signal on a time-frequency plane, comprising: a control signal generation unit configured to generate a control signal indicating whether to perform prioritized mixing that includes amplification of the first signal and attenuation of the second signal; and a gain derivation unit configured to derive a first gain for amplifying the first signal and a second gain for attenuating the second signal based on the control signal, wherein the control signal takes a first value or a second value different from the first value at each point on the time-frequency plane, wherein a band in which the control signal has the first value does not exceed a predetermined range on a frequency axis, wherein the mixing device applies the prioritized mixing to the first signal and the second signal in response to the control signal indicating the first value, and applies simple addition to the first signal and the second signal in response to the control signal indicating the second value, and wherein the control signal generation unit includes: a first frequency-direction processing unit configured to perform first frequency processing with respect to an intensity of the first signal on the time-frequency plane, to obtain a first spectrum representing an absolute amount of the first signal, a second frequency-direction processing unit configured to perform second frequency processing with respect to the first spectrum, wherein the control signal generation unit is configured to generate a second spectrum indicating local changes of the first spectrum based on a difference or a ratio between the first spectrum and a second frequency processing result, and a signal generator configured to generate a first control signal based on the first spectrum, generate a second control signal based on the second spectrum, and output a smaller one of the first control signal and the second control signal as the control signal. a control signal generation unit configured to generate a control signal indicating whether to perform prioritized mixing that includes amplification of the first signal and attenuation of the second signal; and a gain derivation unit configured to derive a first gain for amplifying the first signal and a second gain for attenuating the second signal based on the control signal, wherein the control signal takes a first value or a second value different from the first value at each point on the time-frequency plane, wherein a band in which the control signal has the first value does not exceed a predetermined range on a frequency axis, wherein the mixing device applies the prioritized mixing to the first signal and the second signal in response to the control signal indicating the first value, and applies simple addition to the first signal and the second signal in response to the control signal indicating the second value, and wherein the control signal generation unit includes: a first frequency-direction processing unit configured to perform first frequency processing with respect to an intensity of the first signal on the time-frequency plane, to obtain a first spectrum representing an absolute amount of the first signal, a second frequency-direction processing unit configured to perform second frequency processing with respect to the first spectrum, wherein the control signal generation unit is configured to generate a second spectrum indicating local changes of the first spectrum based on a difference or a ratio between the first spectrum and a second frequency processing result, and a signal generator configured to generate a first control signal based on the first spectrum, generate a second control signal based on the second spectrum, and output a smaller one of the first control signal and the second control signal as the control signal. a first frequency-direction processing unit configured to perform first frequency processing with respect to an intensity of the first signal on the time-frequency plane, to obtain a first spectrum representing an absolute amount of the first signal, a second frequency-direction processing unit configured to perform second frequency processing with respect to the first spectrum, wherein the control signal generation unit is configured to generate a second spectrum indicating local changes of the first spectrum based on a difference or a ratio between the first spectrum and a second frequency processing result, and a signal generator configured to generate a first control signal based on the first spectrum, generate a second control signal based on the second spectrum, and output a smaller one of the first control signal and the second control signal as the control signal.