An inter-lens adjusting method includes: fitting a plurality of fitting portions to a plurality of fitted portions, wherein each of the plurality of fitting portions is provided on the side of a first lens for collimating light emitted from a light emitting device and includes at least one of a convex portion and a concave portion and each of the plurality of fitted portions is provided on the side of a second lens for aggregating the light collimated by the first lens and has a shape to be fitted to a corresponding one of the plurality of fitting portions; and adjusting a position between the first lens and the second lens such that the light emitted from the light emitting device is aggregated into an optical fiber ferrule via the first lens and the second lens.