Optical multiplexer/demultiplexer and manufacturing method thereof

An optical multiplexer/demultiplexer and a manufacturing method thereof are disclosed, wherein a core is arranged on each side of a filter element, and the light is caused to enter the filter element from one of the cores without using a collimator lens. The loss of the transmitted light is reduced by optimizing the axial shift amount of the cores on the two sides. The optical axes of the cores 21, 23 located on both sides of the filter element 26 are displaced by delta. This axial shift amount delta is determined as A(lambda).T.tan theta, where A(lambda) is a predetermined coefficient depending on the wavelength of the incident light, theta the angle between the optical axis of the incident light and the normal to the entrance surface of the filter element, and T the thickness in terms of a medium depending on the refractive indexes of the high refractive index layer and the low refractive index layer making up a multilayer film of the filter element.