To prevent deterioration in image quality in mixing a visible light signal and an infrared light signal to generate a color image signal. A signal processing device includes a mixture ratio creator and a mixer. The mixture ratio creator is configured to create a mixture ratio that is a ratio in mixing a low frequency visible light signal that is a low frequency component of a visible light signal and a low frequency invisible light signal that is a low frequency component of an invisible light signal together on a basis of the low frequency visible light signal and the low frequency invisible light signal. The mixer is configured to mix the low frequency visible light signal and the low frequency invisible light signal together on a basis of the mixture ratio to generate a mixed signal.