A Doherty amplifier, comprising: an input terminal; an output terminal; a splitter connected to the input terminal, the splitter having first and second outputs; a combiner connected to the output terminal, the combiner having first and second inputs; a carrier amplifier including a first input-side two-port network connected to the first output of the splitter, a first amplifier connected to an output of the first input-side two-port network, and a first output-side two-port network connected between an output of a current source of the first amplifier and the first input of the combiner; and a peak amplifier including a second input-side two-port network connected to the second output of the splitter, a second amplifier connected to the output of the second input-side two-port network, and a second output-side two-port network connected between an output of a current source of the second amplifier and the second input of the combiner, wherein the combiner is a parallel-connected load type having a parallel connection of the output-side two-port network of the carrier amplifier and the output-side two-port network of the peak amplifier for the output terminal at a combining point, wherein a load admittance at the combining point is expressed using a complex number, characterised by the Doherty amplifier being so configured that, if the load admittance at the combining point is YL', the F parameters of the output-side two-port network from the current source output of the first amplifier of the carrier amplifier up to the combining point are elements Ac, Bc, Cc, Dc the F parameters of the output-side two-port network from the current source output of the second amplifier of the peak amplifier up to the combining point are elements Ap, Bp, Cp, Dp, the load resistance of the carrier amplifier in the state in which it is not backed off is Roptc, the load resistance of the peak amplifier in the state in which it is not backed off is Roptp, m=1+Roptc/Roptp, and the phase difference between the phase of the current source of the first amplifier of the carrier amplifier and the phase of the current source of the second amplifier of the peak amplifier is θ, within a transmission frequency band, the relationships: Ac × Dp + Bc × Cp + Bc × Dp × YL ′ / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = m × Roptc Δ ⁢ c / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = − Roptc × e j × θ Δ ⁢ p / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = Roptc × e − j × θ, and Ap × Dc + Bp × Cc + Bp × Dc × YL ′ / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = 0 are satisfied, where Δc=Ac × Dc - Bc × Cc and Δp=Ap × Dp - Bp × Cp. an input terminal; an output terminal; a splitter connected to the input terminal, the splitter having first and second outputs; a combiner connected to the output terminal, the combiner having first and second inputs; a carrier amplifier including a first input-side two-port network connected to the first output of the splitter, a first amplifier connected to an output of the first input-side two-port network, and a first output-side two-port network connected between an output of a current source of the first amplifier and the first input of the combiner; and a peak amplifier including a second input-side two-port network connected to the second output of the splitter, a second amplifier connected to the output of the second input-side two-port network, and a second output-side two-port network connected between an output of a current source of the second amplifier and the second input of the combiner, wherein the combiner is a parallel-connected load type having a parallel connection of the output-side two-port network of the carrier amplifier and the output-side two-port network of the peak amplifier for the output terminal at a combining point, wherein a load admittance at the combining point is expressed using a complex number, characterised by the Doherty amplifier being so configured that, if the load admittance at the combining point is YL', the F parameters of the output-side two-port network from the current source output of the first amplifier of the carrier amplifier up to the combining point are elements Ac, Bc, Cc, Dc the F parameters of the output-side two-port network from the current source output of the second amplifier of the peak amplifier up to the combining point are elements Ap, Bp, Cp, Dp, the load resistance of the carrier amplifier in the state in which it is not backed off is Roptc, the load resistance of the peak amplifier in the state in which it is not backed off is Roptp, m=1+Roptc/Roptp, and the phase difference between the phase of the current source of the first amplifier of the carrier amplifier and the phase of the current source of the second amplifier of the peak amplifier is θ, within a transmission frequency band, the relationships: Ac × Dp + Bc × Cp + Bc × Dp × YL ′ / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = m × Roptc Δ ⁢ c / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = − Roptc × e j × θ Δ ⁢ p / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = Roptc × e − j × θ, and Ap × Dc + Bp × Cc + Bp × Dc × YL ′ / Cc × Dp + Cp × Dc + Dc × Dp × YL ′ = 0 are satisfied, where Δc=Ac × Dc - Bc × Cc and Δp=Ap × Dp - Bp × Cp.