The COMET experiment aims to search for the neutrinoless conversion of a muon to an electron in a muonic atom. This experiment utilizes a bunched slow-extracted (bunched-SX) proton beam at 8 GeV from the J-PARC main ring synchrotron. To achieve a sensitivity of $10^{−17}$, the fraction of inter-bunch stray protons, called extinction, must be less than $10^{−10}$ . The 8 GeV proton beam commissioning with a bunched-SX beam for the COMET experiment was performed in May 2021. The extinction was measured by counting all secondary pions in the K1.8BR secondary beamline in the J-PARC Hadron Experimental Facility. After the extinction studies tried out several setups, the most extinction achievable setup was performed with $O(10^{10})$ statistics. This paper reports the extinction measurement and the current analysis status.