Provided is a method of manufacturing an optical device that includes a multicore fiber including a plurality of cores and a fan-in/fan-out device including single-core fibers that are respectively connected to the cores based on a plurality of connection combinations when the multicore fiber is rotated. The method includes: a first step of determining an optical loss for each of the cores while changing the connection combinations between the single-core fibers and the cores; and a second step of selecting one of the connection combinations according to a result of the first step and connecting an end portion of the multicore fiber and an end portion of the fan-in/fan-out device to connect the single-core fibers with the cores based on the one of the connection combinations.