Flat optics based on metasurfaces produce unprecedented two-dimensional planar optical elements that cannot be developed with naturally occurring materials. However, it remains to be shown whether metasurfaces on ultrathin dielectric membranes can be adopted in a broad range of optical elements as flat optics. Here we demonstrate that a fabricated ultrathin metasurface composed of double-sided metal structures on a 100 nm-thick SiNx membrane absorbs infrared rays with a high absorptance of 97.1% at 50.1 THz. This ultrathin metasurface and its fabrication method would be a welcome contribution to a wide range of trailblazing applications, including ultrathin absorbers for imaging and light detection and ranging (LIDAR), directivity control of thermal radiation, and polarization control of vacuum ultraviolet light.