A logarithmic amplifier for preventing an abnormal output from occurring when receiving an excessive input. The logarithmic amplifier is provided with an operational amplifier for logarithmic conversion purposes and a feedback circuit for feeding back an output from the operational amplifier via a resistor connected to an output terminal of the operational amplifier. An element (for example, an element consisting of a Zener diode and a diode connected in series with each other) having an effective resistance which becomes smaller when a voltage above a predetermined value is applied to the element is connected in parallel with a resistor connected to an output terminal of an operational amplifier for logarithmic conversion purposes.