There are disclosed a semiconductor power amplifier and a microwave monolithic integrated circuit which can be reduced in size and cost and which can sufficiently inhibit loop oscillation. The semiconductor power amplifier of the present invention comprises first and second transistors connected in parallel, a capacitor element connected between a signal input terminal and a base terminal of the first transistor, a capacitor element connected between the signal input terminal and a base terminal of the second transistor, and a resistance element connected between the respective base terminals of the first and second transistors. Since the capacitor element and resistance element are disposed, a loop oscillation signal can sufficiently be attenuated on a loop oscillation path. Moreover, in the present embodiment, since miniaturization is possible, MMIC can easily be constituted.