  This article describes the goal and expected sensitivity of the JSNS$^2$-II
experiment at J-PARC Materials and Life Science Experimental Facility (MLF).
The JSNS$^2$-II experiment is the second phase of the JSNS$^2$ experiment
(J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron Source) with two
detectors which are located in 24 m (an existing detector) and 48 m (new one)
baselines to improve the sensitivity of the search for sterile neutrinos,
especially in the low $\Delta m^2$ region, which has been indicated by the
global fit of the appearance mode. The new second detector has a similar
structure as the existing JSNS$^2$ detector, which is already working. To
compensate for the reduction of the neutrino flux due to the distance from the
mercury target, the target mass of the Gd-loaded liquid scintillator which is
the Linear AlkylBenzene (LAB) based liquid scintillator inside the acrylic
vessel is 35 tons. To keep the same photo-coverage of the detector as the first
detector, we will surround the acrylic vessel with 240 PMTs. With this
experimental setup and 5 years (times 1 MW beam power) exposure, the
sensitivity of the JSNS$^2$-II is significantly improved compared to the
current JSNS$^2$, especially in the low $\Delta m^2$ oscillation parameter
region. The JSNS$^2$-II can also confirm or refute the most of the oscillation
parameters' space preferred by the previous experiments with 3 sigma C.L..
Considering these situations and world wide status of the sterile neutrino
searches, we are eager to start the data taking with the two detector
configuration from 2023. The fund to build the second detector was already
secured.
