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 series-connected load type, as a balun which includes one end connected to the output-side two-port networks (C3, P3) of the carrier amplifier and the peak amplifier, and the other end connected to the output terminal, having a series connection of the output-side two-port network of the carrier amplifier and the output-side two-port network of the peak amplifier with the output terminal; wherein a load impedance at the combiner as the balun at the output-side two-port network of the carrier amplifier and at the output-side two-port network of the peak amplifier is expressed using a complex number; and characterised by the Doherty amplifier being so configured that, if the load impedance at the combiner is ZL', if the F parameters of the output-side two-port network from the current source output of the carrier amplifier up until the combiner are Fc (Ac, Bc, Cc, Dc), if the parameters of the output-side two-port network from the current source output of the peak amplifier up until the combiner are Fp (Ap, Bp, Cp, Dp), if the load resistance of the carrier amplifier in the state in which it is not backed off is Roptc, if the load resistance of the peak amplifier in the state in which it is not backed off is Roptp, m=1+Roptc/Roptp, and if the phase difference between the phase of the current source of the carrier amplifier and the phase of the current source of the peak amplifier is θ, then within a transmission frequency band of the receiving signal of the Doherty amplifier, the relationships: Ac × Dp + Bc × Cp + Ac × Cp × ZL ′ / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = m × Roptc; Δ c / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = − Roptc × e j ×; Δ p / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = Roptc × e − j × θ; and Ap × Dc + Bp × Cc + Ap × Cc × ZL ′ / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = 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 series-connected load type, as a balun which includes one end connected to the output-side two-port networks (C3, P3) of the carrier amplifier and the peak amplifier, and the other end connected to the output terminal, having a series connection of the output-side two-port network of the carrier amplifier and the output-side two-port network of the peak amplifier with the output terminal; wherein a load impedance at the combiner as the balun at the output-side two-port network of the carrier amplifier and at the output-side two-port network of the peak amplifier is expressed using a complex number; and characterised by the Doherty amplifier being so configured that, if the load impedance at the combiner is ZL', if the F parameters of the output-side two-port network from the current source output of the carrier amplifier up until the combiner are Fc (Ac, Bc, Cc, Dc), if the parameters of the output-side two-port network from the current source output of the peak amplifier up until the combiner are Fp (Ap, Bp, Cp, Dp), if the load resistance of the carrier amplifier in the state in which it is not backed off is Roptc, if the load resistance of the peak amplifier in the state in which it is not backed off is Roptp, m=1+Roptc/Roptp, and if the phase difference between the phase of the current source of the carrier amplifier and the phase of the current source of the peak amplifier is θ, then within a transmission frequency band of the receiving signal of the Doherty amplifier, the relationships: Ac × Dp + Bc × Cp + Ac × Cp × ZL ′ / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = m × Roptc; Δ c / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = − Roptc × e j ×; Δ p / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = Roptc × e − j × θ; and Ap × Dc + Bp × Cc + Ap × Cc × ZL ′ / Cc × Dp + Cp × Dc + Cc × Cp × ZL ′ = 0 are satisfied, where Δc=Ac × Dc - Bc × Cc and Δp=Ap × Dp - Bp × Cp.