BACKGROUND: Pediatric patients, especially in the preverbal stage, cannot self-report intensity of pain therefore several validated observational tools, including the Face, Legs, Activity, Cry, Consolability (FLACC) Behavioral Scale, have been used as a benchmark to evaluate pediatric pain. Unfortunately, this scale is currently unavailable in Japanese, precluding its widespread use in Japanese hospitals. OBJECTIVES: To translate and verify the validity and reliability of the Japanese version of the FLACC Behavioral Scale. METHOD: Back-translation was first conducted by eight medical researchers, then an available sample of patients at the University of Tsukuba Pediatric Intensive Care Unit (from May 2017 to August 2017) was enrolled in a clinical study. Two researchers evaluated the validity of the translated FLACC Behavioral Scale by weighted kappa coefficient and intraclass correlation coefficients (ICC). Observational pain was simultaneously measured by the visual analog scale (VAS obs) and reliability was evaluated by correlation analysis. RESULT: The original author approved the translation. For the clinical study, a total of 121 observations were obtained from 24 pediatric patients. Agreement between observers was highly correlated for each of the FLACC categories (Face: kappa = 0.85, Leg: kappa = 0.74, Activity: kappa = 0.89, Cry: kappa = 0.93, Consolability: kappa = 0.93) as well as the total score (Total: kappa = 0.95,). Correlation analysis demonstrated a good criterion validation between the FLACC scale and the VAS obs. (r = 0.96). CONCLUSION: Our Japanese version of the FLACC Behavioral Scale shows high validity and reliability.