  We present the performances of a superconducting nanowire single-photon
detector (SNSPD) in an ultralow-temperature region from 16 mK to 4 K. The
system detection efficiency of the SNSPD showed saturation in the bias-current
and temperature dependences, and reached the considerably high value of 15% for
100 Hz dark count rate below 1.4 K at a wavelength of 1550 nm even without an
optical cavity structure. We found that the dark count exists even at 16 mK and
black body radiation becomes its dominant origin in the low temperatures for
fiber-coupled devices.
