A fixing instrument includes a first frame member (2) and a second frame member (3) that clamp and fix an aperture array structure. At least any of conditions of Expression 1: A1+C>B1 and Expression 2: A2+C>B2 is satisfied where A1 represents a distance between a first reference surface (22a) and a first inner peripheral fitting face (21a), A2 represents a distance between the first reference surface (22a) and a first outer peripheral fitting face (21b), B1 represents a distance between a second reference surface (32a) and a second inner peripheral fitting face (31a), B2 represents a distance between the second reference surface (32a) and a second outer peripheral fitting face (31b), and C represents a thickness of an outer peripheral section (102) of the aperture array structure.