In this paper we present the results of the research on the superconducting NbN-ultrathin-film singlephoton detectors (SSPD) which are capable to detect single quanta in middle IR range. The detection mechanism is based on the hotspot formation in quasi-two-dimensional superconducting structures upon photon absorption. Spectral measurements showed that up to 5.7 μm wavelength (52 THz) the SSPD exhibits single-photon sensitivity. Reduction of operation temperature to 1.6 K allowed us to measure quantum efficiency of ~1% at 60 THz. Although further decrease of the operation temperature far below 1 K does not lead to any significant increase of quantum efficiency. We expect that the improvement of the SSPD's performance at reduced operation temperature will make SSPD a practical detector with high characteristics for much lower THz frequencies as well. Index Terms- Superconducting single photon detector, Hot spot, Infrared range, ultrathin films.