The invention is to provide an optical loss measuring method and an optical loss measuring apparatus capable of efficiently measuring an optical loss occurring to a target for measurement. The other optical loss measurement method comprising the steps of inputting light outputted from a light source to a target for measurement to thereby measure an output level of the target for measurement with the use of an optical power meter, maintaining the output level of the light source at a predetermined fixed value, and finding the optical loss occurring to the target for measurement on the basis of the output level of the light source maintained at the fixed value for use as the reference value.