<p>An optical device, wherein when the multi-layer film (56) side surface of a filter member (20) is defined as a first surface (70) and the quartz substrate (54) side surface thereof is defined as a second surface (72), and the inner wall surface of a slit (18) opposed to the first surface (70) of the filter member (20) is defined as a first inner wall surface (74) and the inner wall surface thereof opposed to the second surface (72) of the filter member (20) is defined as a second inner wall surface (76), one or more of the first inner wall surface (74) and the second inner wall surface (76) of the slit (18) and the second surface (72) of the filter member (20) are positioned non-parallel with the first surface (70) of the filter member (20).</p>