What each includes, concentration, currency exposure, cost, overlap, and which investor each may fit.
An all-country fund spreads across many countries including the US and Japan, while an S&P 500 fund concentrates in large US companies; the choice is between global diversification and a deliberate US bet — and holding both mostly duplicates US exposure.
An all-country fund holds companies across developed markets, emerging markets, and Japan, weighted by market size. Because US companies are the largest share of global market value, the US is already the biggest single component of an all-country fund — often well over half. An S&P 500 fund holds only large US companies.
So the practical difference is the non-US portion: an all-country fund adds Japan, Europe, other developed markets, and emerging markets that the S&P 500 leaves out. If you hold both, you are mostly buying the same US large caps twice, with a smaller genuine addition of non-US exposure from the all-country fund.
Choose an all-country fund if you want a single, hands-off, globally diversified equity holding and prefer not to bet on one country. Choose an S&P 500 fund if you deliberately want US concentration and accept the single-country risk. There is no universally correct answer, and recent US outperformance is not a reliable reason — it reflects one historical regime.
The overlap warning matters most: buying both an all-country and an S&P 500 fund does not double your diversification; it just overweights the US relative to the rest of the world. If you want more US than global weights imply, that is a valid choice — just make it deliberately, not by accidentally stacking overlapping funds.
Brokerage (NISA/iDeCo) · Rakuten Securities
A leading low-cost brokerage for NISA and index-fund investing, integrated with Rakuten points and Rakuten Bank.
Fees: Many domestic funds and trades are low- or no-commission — verify current fee schedule.
Brokerage (NISA/iDeCo) · SBI Securities
Japan’s largest online brokerage by accounts, with a very broad low-cost fund lineup and full NISA/iDeCo support.
Fees: Many trades and funds are low- or no-commission — verify current fee schedule.
Brokerage (NISA/iDeCo) · Monex
An established online brokerage often chosen for US-stock access and research tools, with NISA and iDeCo support.
Fees: Commissions vary by product; some funds/trades are low-cost — verify current schedule.
This is education, not a recommendation. Most beginners research low-cost, broadly diversified index funds — for example all-country (全世界株式) or S&P 500 trackers — rather than picking individual stocks, because low fees and diversification are within your control while returns are not. Understand that values fall as well as rise, match the risk to your time horizon, and never invest money you may need soon.