An optical power meter including a photodiode and an amplifier circuit is provided. The photodiode is for generating a current based on the power of a light beam incidents thereon. The amplifier circuit is for converting the current into a voltage and includes an amplifier, a capacitor, a plurality of resistors and a relay. The amplifier includes an inverting input terminal and an output terminal. The capacitor has a first terminal connected with the inverting input terminal and a second terminal connected with the output terminal. Each of the plurality of resistors has a first terminal connected with the inverting input terminal. The relay has a first terminal connected with the output terminal. The relay selectively connects one of the resistors with the output terminal so that the value of the voltage outputted by the output terminal is within a predetermined range. The on-resistance of the relay is not larger than 10 Ω.