The article explores errors in the Japanese Consumer Price Index caused by lower-level substitution within items. The study uses micro data from the Retail Price Survey to assess this bias. Findings show that the Japanese CPI has a unique lower-level substitution bias compared to the U.S. CPI. While the one-specification for one-item policy helps maintain price homogeneity, it also weakens price representativeness, requiring additional data sources for accurate assessments.