A power amplifier includes: a first multi-finger FET formed on a semiconductor substrate; a second multi-finger FET formed on the semiconductor substrate; a first temperature detector which detects a channel temperature of the first FET; a second temperature detector which detects a channel temperature of the second FET; a third temperature detector which detects a temperature of the semiconductor substrate; a first detection circuit detecting a difference between an output of the first temperature detector and an output of the third temperature detector and converting the difference to thermoelectromotive force; a second detection circuit detecting a difference between an output of the second temperature detector and the output of the third temperature detector and converting the difference to thermoelectromotive force; and a comparator comparing outputs of the first and second detection circuits with each other to turn on one of the first and second switches and turn off the other.