  We report the discovery of an SN1988Z-like type IIn supernova KISS15s found
in a low-mass star-forming galaxy at redshift z=0.038 during the course of the
Kiso Supernova Survey (KISS). KISS15s shows long-duration optical continuum and
emission line light curves, indicating that KISS15s is powered by a continuous
interaction between the expanding ejecta and dense circumstellar medium (CSM).
The H$\alpha$ emission line profile can be decomposed into four Gaussians of
narrow, intermediate, blue-shifted intermediate, and broad velocity width
components, with a full width at half maximum of $\lesssim 100$, $\sim 2,000$,
and $\sim 14,000$ km s${}^{-1}$ for the narrow, intermediate, and broad
components, respectively. The presence of the blue-shifted intermediate
component, of which the line-of-sight velocity relative to the systemic
velocity is about $-5,000$ km s${}^{-1}$, suggests that the ejecta-CSM
interaction region has an inhomogeneous morphology and anisotropic expansion
velocity. We found that KISS15s shows increasing infrared continuum emission,
which can be interpreted as hot dust thermal emission of $T \sim 1,200$ K from
newly formed dust in a cool, dense shell in the ejecta-CSM interaction region.
The progenitor mass-loss rate, inferred from bolometric luminosity, is $\dot{M}
\sim 0.4 M_{\odot} \text{yr}^{-1} (v_{w}/40 \text{km}~\text{s}^{-1})$, where
$v_{w}$ is the progenitor's stellar wind velocity. This implies that the
progenitor of KISS15s was a red supergiant star or a luminous blue variable
that had experienced a large mass-loss in the centuries before the explosion.
