What Is a Money Market Fund?

5 分で読める· 最終更新日: 2026-08-14· 編集チームによる作成、以下のソースをもとに確認済み ·

簡単な答えA money market mutual fund is an investment fund that holds very short-term, high-quality debt (like Treasury bills) and pays a yield that moves with interest rates. It is NOT a bank account and NOT federally insured, while a money market deposit account (MMDA) at a bank IS insured like any deposit. The names are nearly identical on purpose-defying levels; the insurance is the difference that matters.

なぜ大切なのか

When rates are decent, money market funds are where brokerages park cash and often out-yield savings accounts. Knowing the fund/account distinction, and what insurance covers, is the difference between an informed choice and a surprise.

ステップ

Know the two look-alikes

Money market DEPOSIT account: a bank product, FDIC/NCUA-insured within limits, rate set by the bank. Money market MUTUAL FUND: an investment bought through a brokerage, yield from short-term debt markets, not insured.

Know the fund families

Government funds hold Treasuries and government-backed paper (the most conservative flavor); prime funds add corporate short-term debt for slightly more yield; municipal funds trade yield for tax treatment.

Understand the $1 share convention

These funds aim to keep each share worth $1 and pay interest as dividends. Extremely rare stress events ('breaking the buck', 2008) are why regulation tightened; stability is the design goal, not a guarantee.

Match the tool to the job

Emergency fund you must never lose: insured accounts win. Cash at a brokerage waiting to be used, or a yield-chasing cushion beyond the insured fund: a government money market fund is the standard tool.

Same $10,000, three parking spots (illustrative math, not quotes)

Big-bank savings at 0.1% APY$10 interest in a year

High-yield savings at 4.0% APYabout $407 with monthly compounding

Government money market fund yielding 4.5%about $450: uninsured, and the yield floats daily

よくある失敗

Assuming 'money market' means insured. Only the bank deposit version is.
Parking next month's rent in any investment product. Timing risk is real even in conservative funds.
Chasing a prime fund's extra 0.1% for an emergency fund. The safety margin is worth more than the sliver.
Forgetting yields float. Last month's 4.5% is a snapshot, not a promise.

次にすること

Compare what your cash earns today using the Interest Rate Comparison tool, then read the full account-types guide before moving anything.

ここで計算してみよう: Interest Rate Comparison The same balance at five different rates, one year and five years out.

具体的な計算例

Balance: $10,000 · Rate 1 (%): 0.1% · Rate 2 (%): 1% · Rate 3 (%): 3% · Rate 4 (%): 4% · Rate 5 (%): 5%

Balance compared$10,000

Gap between 0.1% and 5.0%$501.61 in year one

Rates float; treat these as comparison math, not quotes. The point survives any rate era: the branch-average account and the competitive account differ by real money on the same balance.

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出典

SEC: Money market funds (Investor.gov) · FDIC: What's covered by deposit insurance