A signal amplifier includes an inverting amplification circuit, a first switching element, a second switching element, and a control section. The inverting amplification circuit includes a first voltage terminal, a second voltage terminal, an inverting input terminal, an output terminal, a first protected switching element, and a second protected switching element. The control section controls such that when an overcurrent has flowed in the first voltage line, the first and second protected switching elements are switched to a non-conducting state after switching the first switching element in a conducting state and switching the second switching element in a non-conducting state, and when an overcurrent has flowed in the second voltage line, the first the second protected switching elements are switched to a non-conducting state after switching the first switching element in a non-conducting state and switching the second switching element in a conducting state.