  Integral Field Spectroscopy (IFS) is an observational method to obtain
spatially resolved spectra over a specific field of view (FoV) in a single
exposure. In recent years, near-infrared IFS has gained importance in observing
objects with strong dust attenuation or at high redshift. One limitation of
existing near-infrared IFS instruments is their relatively small FoV, less than
100 arcsec$^2$, compared to optical instruments. Therefore, we have developed a
near-infrared (0.9-2.5 $\mathrm{\mu}$m) image-slicer type integral field unit
(IFU) with a larger FoV of 13.5 $\times$ 10.4 arcsec$^2$ by matching a slice
width to a typical seeing size of 0.4 arcsec. The IFU has a compact optical
design utilizing off-axis ellipsoidal mirrors to reduce aberrations. Complex
optical elements were fabricated using an ultra-precision cutting machine to
achieve RMS surface roughness of less than 10 nm and a P-V shape error of less
than 300 nm. The ultra-precision machining can also simplify alignment
procedures. The on-sky performance evaluation confirmed that the image quality
and the throughput of the IFU were as designed. In conclusion, we have
successfully developed a compact IFU utilizing an ultra-precision cutting
technique, almost fulfilling the requirements.
