The near-infrared absorption line Fe I 15648 Å, which has a Landé g-factor of 3, shows a particularly large Zeeman splitting. We regularly take full-disk polarization maps of the Sun in the Fe I 15648 Å line (as well as the He I 10830 Å line) with an infrared spectropolarimeter installed at the Solar Flare Telescope of the National Astronomical Observatory of Japan (NAOJ). It is known that weak, mostly horizontal magnetic fields are ubiquitously distributed in the quiet regions of the Sun, while the strong magnetic fields are concentrated in active regions and network boundaries. The weak horizontal field has not been sufficiently investigated due to the difficulty of such observations. The polarization maps in Fe I 15648 Å show the magnetic field strength at each pixel, regardless of the filling factor, so we can easily isolate the weak horizontal field signals from strong magnetic field ones using the Stokes V profiles of the Fe I 15648 Å line. Here we present instrumental aspects and observational results of solar near-infrared full-disk polarimetry. We highlight the weak horizontal field inferred from Fe I 15648 Å.