An amplifier includes: a circuit pattern providing a plurality of signal paths having different lengths; a transistor chip; a plurality of pads of transistor cells, the pads being electrically connected to the circuit pattern and being arranged on the transistor chip; a plurality of the transistor cells; a plurality of transmission lines for connecting each of the plurality of pads and each of the plurality of transistor cells, the transmission lines being arranged on the transistor chip, and a plurality of harmonic processing circuits each connected to each of the plurality of transmission lines and arranged on the transistor chip. The plurality of harmonic processing circuits each has a capacitor and an inductor, and a product of the capacitance of the capacitor and the inductance of the inductor is made constant in each of the plurality of harmonic processing circuits.