PURPOSE: To measure optical loss with good accuracy by stabilizing the optical loss of a light transmitting medium, by using an optical waveguide to a part of the light path of a coherent light source and a light transmitting medium to be measured while applying lasting mechanical vibration to the optical waveguide during the measuring time of the optical loss of the light transmitting medium to be measured.

CONSTITUTION: In measuring optical loss by guiding the output beam from a laser beam source 101 to an optical fiber while applying the beam from an optical fiber 102 to a light transmitting medium 103 to be measured, the optical fiber is wound in a ring shape and, when this ring shaped optical fiber is repeatedly deformed from circular shape to an oval shape or from a vertically long oval shape to a horizontally long oval shape, the variation of the optical loss of the light transmitting medium 103 to be measured becomes extremely low and the reproducibility of measurement is increased. In order to apply mechanical vibration to the optical fiber 102, a part of the optical fiber 102 is made movable and this part is vibrated to the direction shown by the arrow. The vibration is performed, for example, by a crank 104 converting the circular motion of a motor to linear motion. If vibration frequency is made larger than the frequency response band of an optical loss measuring instrument, effect is developed.