PROBLEM TO BE SOLVED: To provide a manufacturing method of an optical waveguide, the optical waveguide, and a reflection surface forming method that can form a plurality of reflection surfaces at a time and increase the forming accuracy of the reflection surfaces.SOLUTION: The manufacturing method of an optical waveguide of the present invention includes an arrangement process and a forming process. In the arrangement process, a movable mask 300 including a plurality of openings 310 that are arranged in parallel along the X direction and have a part whose length in the X direction varies continuously along the Y direction is disposed on one surface side of an optical waveguide 1A including a plurality of core parts 14 arranged in parallel at an interval in the X direction. In the forming process, laser beams LL are radiated to the optical waveguide 1A via the openings 310 in the movable mask 300, and the movable mask 300 is moved in the X direction, thereby forming a plurality of notches 70 having a mirror 7
