Context :

Aims : We announce the discovery of a microlensing planetary system , in which a sub-Saturn planet is orbiting an ultracool dwarf host .

Methods : We detect the planetary system by analyzing the short-timescale ( t _ { 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 .

Results : 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 _ { 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 _ { p } = 0.18 ^ { +0.29 } _ { -0.10 } ~ { } M _ { J } , and the mass of the host , M _ { 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 .

Conclusions :