An amplifier includes: a first transistor that includes a first main electrode, a second main electrode, and a first control electrode, a first input signal being input to the first main electrode, a first output signal being output from the second main electrode; a reference potential line that is disposed on a signal line connected to the second main electrode with an insulator interposed therebetween; a first capacitor that is disposed between the first control electrode and the reference potential line; and a first phase shifter configured to shift a phase of a first return current such that the phase of the first return current which flows from the second main electrode to the first control electrode via the reference potential line and the first capacitor has a phase difference, which is greater than 90 degrees and less than 270 degrees, from the phase of the first input signal.