An optical device of the present invention comprises an image sensing device, a package for accommodating the image sensing device, the package including an opening of a square configuration as viewed in plan, and an optical low-pass filter having square-shaped major surfaces and fixed in the opening by means of a UV-curable adhesive. The optical low-pass filter comprises a 45-degree splitting birefringent plate which is machine-cut in such a manner as to split an incident beam in a direction of 45 degrees and to form a two-point split pattern. The optical low-pass filter is formed with a color compensation filter and/or an anti-reflection filter, and further provided with a UV transmission area (e.g. notches, angular grooves) for curing the UV-curable adhesive.