PURPOSE: To achieve a broader service wavelength of a halfmirror while enabling the use of a thin film with a low strength, by fixing a thin film having functions of a mirror and a lens on the surface of a honeycomb core.

CONSTITUTION: A halfmirror has a honeycomb core 4 with the surface thereof made planar by electric discharge and electrochemical machining supported on a mount ring 11 and a ring 7 with a thin film 6 stuck thereon is pressed on the surface of the core 4 to be fixed with rings 8 and 9. The thin film material employs polyethylene terephthalate, polyethylene or the like. Thus, the vibrating part can be reduced by fixing the thin film 6 on the surface of the honeycomb 4 thereby achieving a broader wavelength range. This also enables the use of the thin film 6 even with a low strength made of polyethylene, ethylene or the like.