A first transistor (2a), a second transistor (2b), a third transistor (2c) and a fourth transistor (2d) are provided. A first transistor (2a) amplifies a first I signal VIP inputted from a first input terminal (1a). A second transistor (2b) amplifies a first Q signal VQP inputted from a second input terminal (1b). A third transistor (2c) amplifies a second I signal VIN when the second I signal VIN is inputted from a third input terminal (1c), the second I signal VIN forming a differential signal with the first I signal VIP. A fourth transistor (2d) amplifies a second Q signal VQN when the second Q signal VQN is inputted from a fourth input terminal (1d), the second Q signal VQN forming a differential signal with the first Q signal VQP.