The KOTO II is a next-generation experiment to measure the branching ratio of $K_L\to \pi^0\nu\bar{\nu}$ with 30-GeV proton beam at J-PARC. The KOTO II is a successor of the currently running KOTO experiment. We plan to expand the hadron experimental facility at J-PARC, and construct a new beamline of KOTO II there. The extraction angle of the $K_L$ is 5 degrees, smaller than that in KOTO to have more $K_L$ with a harder momentum spectrum.
The KOTO II detector is being designed with a 12-m signal decay region
and a 3-m diameter calorimeter to have more signal acceptance. The expected numbers of signal and background events are 35 and 40, respectively. Here, the Standard Model value of the branching ratio and $3\times 10^7$-s running time are assumed. The signal can be observed with $5\sigma$ significance. The design, current developments, and the expected sensitivity of KOTO II will be reported.

