[en] In the Japanese Spallation Neutron Source (JSNS) of Japanese Proton Accelerator Research Complex (J-PARC), proton beam with a high power such as 1 MW is induced to the target consisted of mercury. As increasing in the beam power, the damage of the target becomes serious. Reduction of the peak current density at the target is a key for a constant beam operation. For reduction of the peak current density, a non-linear beam optics using octupole magnets has been developed. In order to adjust beam optics efficiently, a beam tuning tool was developed based on the SAD code. It was found that calculated beam profile with the excitation of octupole magnets was in good agreement with the experimental result for 0.8 MW beam. By using octupole magnet, a considerable flat distribution can be obtained. The peak current density can be reduced as much as 30%, which mitigates 76% of the pitting damage at the target. (author)