The Bank of Japan’s (BOJ) Monetary Base table for September 2026, released on October 2, showed the average monetary base down 15.2% from a year earlier. The annual decline was 15.7% in August and 13.8% in July. Unless otherwise indicated, the table’s headline figures are year-on-year changes in average amounts outstanding during each period. The September rate therefore does not compare the stock at the end of September with the stock a year earlier. The release reports end-period balances separately. Those measures answer different questions and should not be substituted for one another when describing the central bank’s liabilities.

The monetary base brings together banknotes in circulation, coins in circulation and current-account balances held at the BOJ. It is a measure of central-bank liabilities associated with cash and reserve accounts; it is not the broad money supply and is not the amount of loans banks have extended to households and firms. In September, average banknotes in circulation were down 2.0% year on year and coins were down 1.2%. BOJ current-account balances declined 18.3%. Reserve balances fell 17.3%, but they are a component within current-account balances and must not be added a second time to the monetary-base total. The component rates refer to distinct, though related, balance-sheet categories.

The source PDF reports average amounts outstanding in units of 100 million yen. The average monetary-base amount was 5,328,464 such units in September and 5,430,070 in August, equivalent to ¥532.8464 trillion and ¥543.0070 trillion, respectively. A separate table gives end-period stocks of 5,354,214 units for September and 5,437,652 for August. An average balance reflects levels across the month, whereas an end-period amount is a stock on a particular date. The source provides these amounts alongside growth rates, but they have different definitions and reference points. The comparison between September and August amounts is a month-to-month level comparison, not the year-on-year rate above.

The year-on-year rates measure changes in each reported monetary-base component from the same month a year earlier; they are not month-on-month changes. The BOJ also publishes a seasonally adjusted rate for the monetary base, annualized from the previous month. That measure was minus 10.2% in September, compared with minus 28.8% in August. An annualized monthly rate expresses the seasonally adjusted change over one month as if it continued for a year; it is not the actual change over the past 12 months. Seasonality and the choice of comparison period can cause the two rates to differ in size and direction. Reading them as a single like-for-like series would obscure what each one measures.

The release documents the September monetary-base figures, but it does not by itself establish that a particular BOJ policy decision, asset-purchase operation or bank-lending choice caused the decline. The table defines the reported measures and their scope; it does not provide a causal decomposition of the movements. A smaller monetary base does not imply that broad money or economic activity contracted by the same percentage. Assessing those questions would require other balance-sheet items, the coverage of reserve accounts, credit and price indicators, and the policy framework. This article stays with the September release, distinguishes average from end-period amounts and separates the year-on-year measure from the annualized monthly rate. It does not turn one monthly observation into a forecast or a broader policy conclusion.