A signal amplification device includes: a signal generator which divides an input signal so as to generate a plurality of division signals having a specified amplitude and different phases; a plurality of amplifiers which amplify the plurality of division signals by using a field effect transistor, respectively; transmission lines through which the division signals pass, the division signals being amplified with individual amplifiers of the plurality of amplifiers, the transmission lines have different lengths and no isolation is provided between ends of the transmission lines; and a detector which performs a comparison between gate currents input to the individual amplifiers to detect whether an input of the division signal to any of the plurality of the amplifiers exceeds a saturation level.