Our investment securities, consisting of debt instruments at amortized cost, debt instruments at fair value through other comprehensive income and equity instruments at fair value through other comprehensive income, totaled ¥34,082,730 million at September 30, 2023, an increase of ¥6,487,132 million, or 24%, from ¥27,595,598 million at March 31, 2023. The increase in our investment securities was primarily due to increases in our holdings of U.S. Treasury and other U.S. government agency bonds and mortgage-backed securities.
Our bond portfolio is principally held for asset and liability management purposes. It mostly consisted of Japanese government bonds, U.S. Treasury securities and bonds issued or guaranteed by foreign governments, government agencies or official institutions.
Our debt instruments at amortized cost amounted to ¥290,051 million at September 30, 2023, an increase of ¥54,484 million, or 23%, from ¥235,567 million at March 31, 2023, primarily due to increases in our holdings of Japanese government bonds and Japanese municipal bonds.
Domestic debt instruments at fair value through other comprehensive income amounted to ¥11,500,722 million at September 30, 2023, a decrease of ¥110,239 million, or 1%, from ¥11,610,961 million at March 31, 2023. The decrease was primarily due to a decrease in our holdings of Japanese government bonds. As for our foreign debt instruments at fair value through other comprehensive income, we had ¥17,045,084 million of foreign debt instruments at September 30, 2023, which was an increase of ¥5,844,622 million, or 52%, from ¥11,200,462 million at March 31, 2023. Most of our foreign debt instruments, including mortgage-backed securities, are issued or guaranteed by foreign governments, government agencies or official institutions. The increase was primarily due to increases in our holdings of U.S. Treasury and other U.S. government agency bonds and mortgage-backed securities.
We had ¥4,071,827 million of domestic equity instruments and ¥1,175,046 million of foreign equity instruments at September 30, 2023, for which we made an irrevocable election at initial recognition to present subsequent changes in fair value in other comprehensive income under IFRS 9 “Financial Instruments.” Our domestic equity instruments, which consisted principally of publicly traded Japanese stocks and included common and preferred stocks issued by our customers, increased by ¥483,863 million, or 13%, from ¥3,587,964 million at March 31, 2023. Net unrealized gains on our domestic equity instruments increased by ¥509,582 million, or 23%, from ¥2,250,969 million at March 31, 2023 to ¥2,760,551 million at September 30, 2023. The increase was primarily due to an increase in the fair value of publicly traded Japanese stocks. Net unrealized gains on our foreign equity instruments increased by ¥173,298 million, or 25%, from ¥697,032 million at March 31, 2023 to ¥870,330 million at September 30, 2023, mainly reflecting favorable conditions in overseas stock markets.
We have no transactions pursuant to repurchase agreements, securities lending transactions or other transactions involving the transfer of financial assets with an obligation to repurchase such transferred assets that are treated as sales for accounting purposes.