  We announce the discovery of a microlensing planetary system, in which a
sub-Saturn planet is orbiting an ultracool dwarf host. We detect the planetary
system by analyzing the short-timescale ($t_{\rm E}\sim 4.4$~days) lensing
event KMT-2018-BLG-0748. The central part of the light curve exhibits asymmetry
due to the negative deviations in the rising part and the positive deviations
in the falling part. We find that the deviations are explained by a binary-lens
model with a mass ratio between the lens components of $q\sim 2\times 10^{-3}$.
The short event timescale together with the small angular Einstein radius,
$\theta_{\rm E}\sim 0.11$~mas, indicate that the mass of the planet host is
very small. The Bayesian analysis conducted under the assumption that the
planet frequency is independent of the host mass indicates that the mass of the
planet is $M_{\rm p}=0.18^{+0.29}_{-0.10}~M_{\rm J}$, and the mass of the host,
$M_{\rm h}= 0.087^{+0.138}_{-0.047}~M_\odot$, is near the star/brown dwarf
boundary, but the estimated host mass is sensitive to the assumption about the
planet hosting probability. High-resolution follow-up observations would lead
to revealing the nature of the planet host.
