First and second dielectric multilayer film filters are disposed along a straight optical axis and inclined about 45.degree. with respect thereto. The second filter is rotated about the optical axis by a predetermined angle with respect to the first filter so as to divide a path of light incident upon or emitting from the first filter into a first path of light transmitting through the first and second filters, and a second path of light reflected by the first filter. The second path is orthogonal to the first path. The apparatus is used for separating or synthesizing light signals.