An optical power measuring apparatus for measuring a power level of an optical signal comprised of a light intensity modulated signal, received by a light receiving element is provided. In the apparatus, a first signal path including a first voltage detection circuit and a first amplifier circuit supplies a processor with a first signal having a voltage proportional to a current flowing through a light receiving element (PD), and a second path including a TIA, a DC detection circuit and a second amplifier circuit supply the processor with a second signal. The second signal corresponding to an envelope of a voltage from the TIA. The processor compares the level of the first signal with a predetermined set value to select an optical power value retrieved based on the first signal when the level of the first signal is equal to or higher than the set value, and to select an optical power value retrieved based on the second signal when lower than the set value, and outputs the selected optical power value as a measured optical power value Pout. An output signal of the TIA can be supplied to an external device, so that the optical power measuring apparatus can measure an optical power level while serving as an optical signal receiving apparatus.