A class-AB power amplifier according to the present embodiment includes an amplifying element whose power supply voltage is expressed as Vdc and whose maximum current is expressed as Imax, a conduction angle θo of the amplifying element being more than π(rad) and less than 2·π(rad), and load impedance of a fundamental wave being expressed as Z1=R1+j·X1, load impedance of a 2nd harmonic being expressed as Z2=R2+j·X2, and load impedance of a 3rd harmonic being expressed as Z3=R3+j−X3 which are observed from a dependent current source of an equivalent circuit of the amplifying element, and a relationship between variables X1 and R1 is set to −0.5·R1