According to one embodiment, an amplifier includes: a transistor; a ground circuit connected to a ground terminal of the transistor; a first capacitor connected between an output terminal of the transistor and the ground circuit; a first inductor connected to the output terminal of the transistor; a second capacitor connected between the first inductor and the ground circuit; a bias circuit connected between the first inductor and the ground circuit; a first circuit connected to the output terminal of the transistor, the first circuit including a second inductor and a third capacitor connected in series to the second inductor; a fourth capacitor connected between the first circuit and a load circuit; a fifth capacitor connected between an output terminal of the first circuit and the ground circuit; and a third inductor connected between a terminal on a load circuit side of the fourth capacitor and the ground circuit.