question: Background: To control the COVID−19 outbreak in Japan, sports and entertainment events were canceled throughout Japan for two weeks from February 26 through March 19. That policy has been designated as voluntary event cancellation (VECSC). Object: This study assesses the VECSC effectiveness using predicted outcomes. Method: A simple susceptible−infected−recovery model was applied to data of patients with symptoms in Japan during January 14 through March 25. The respective reproduction numbers before VECSC (R), during VECSC (Re), and after VECSC (Ra) were estimated. Results: Results suggest R before VECSC as 1.992 and its range was [1.892, 2.088]. Moreover, Re was estimated as 1.125 [0.933, 1.345] and Ra was estimated as 3.083 [2.375, 4.067]. Discussion and Conclusion: Results demonstrated that VECSC can reduce COVID−19 infectiousness considerably, but the value of R rose to exceed 2.5 after VECSC.