The QUBIC instrument is made of a cryostat cooled down to 4K using pulse-tubes. The cryostat is open to the sky with a 45 cm diameter window made of high-density polyethylene (HDPE) providing an excellent transmission and mechanical stiffness. Right after the window, filters ensure a low thermal load inside the cryostat and a rotating Half-Wave-Plate (HWP) modulates the polarization. Then, a polarizing grid selects one of the two polarization angles w.r.t the instrument. An array of 400 corrugated horns (called « primary horns » designed to be efficient throughout the 150 and 220 GHz bands with a ≈ 13 degrees FWHM at 150 GHz) selects the baselines observed by QUBIC. These primary horns are immediately followed by back-horns re-emitting the signal inside the cryostat towards an « optical combiner » which is simply a telescope that combines on the focal plane the images of each of the secondary horns in order to form interference fringes. Before the focal plane, a dichroic plate splits the signal into its 150 and 220 GHz components that are each imaged on a focal plane equipped with 1024 Transition-Edge-Sensors (TES) from which 992 are exposed to the sky radiation (blind ones are used for systematics studies) cooled down to 320 mK and read using a multiplexed cryogenic readout system based on SQUIDs and SiGe ASIC operating at 4K.