PURPOSE: To improve the accuracy of measurement in a measuring method for optical waveguide transmission loss.

CONSTITUTION: A triangular prism 40 is mounted on the surface of the core layer 13 of an optical waveguide 10 separated by the distance L from the outgoing end face 15 of the optical waveguide 10. The outgoing light of a light source 20 is projected to one face 41 of the triangular prism 40 at an incidence angle that it is totally reflected on the boundary surface 42 between the triangular prism 40 and the core layer 13, and the light power P0 of the reflected light on the boundary surface 42 after being emitted from the other face of the triangular prism 40 is measured by the first photo-detector 51. The outgoing light of the light source 20 is projected to one face 41 of the triangular prism 40 at an incidence angle that it is not totally reflected on the boundary surface 42, the light power P1 of the reflected light on the boundary surface 42 after being emitted from the other face 43 of the triangular prism 40 is measured by the first photo-detector 51, and the light power P2 of the outgoing light from an outgoing end face 15 after being transmitted to the core layer 13 through the boundary surface 42 is measured by the second photo-detector 52.