Provided are an amplifying device and signal processing device for mitigating delay and distortion of a power supply voltage used in drain modulation. An amplifying device (10) is an amplifying device for performing drain modulation, and is provided with a print substrate (400) comprising a first main surface (400a) and a second main surface (400b) which are mutually opposed, an amplifying circuit (100) arranged upon a first main surface (400a), and a modulation power supply circuit (200) for supplying a variable power supply voltage for performing the drain modulation to the amplifying circuit (100). The modulation power supply circuit (200) comprises an output unit (250) for outputting the power supply voltage. The amplifying circuit (100) comprises an input unit (150) to which the power supply voltage is supplied. The output unit (250) is positioned upon the second main surface (400b) side of the print substrate (400), and is connected to the input unit (150) via a conductor extending through the print substrate (400).