Country and currency concentration, sector differences, familiarity bias, matching yen liabilities, and the role each can play.
A Japan-only portfolio concentrates you in one country, one currency, and an export/industrial-heavy market; a global portfolio spreads across many economies and sectors but adds currency and geopolitical dispersion — most investors use global as the core, with a Japan tilt optional.
The core difference is breadth. A Japan-only portfolio ties your outcome to one economy, one political and regulatory system, and predominantly the yen. A global portfolio holds many economies and currencies, so no single country’s fortunes dominate. In terms of numbers of economies and currencies, global is simply more diversified.
Sector mix follows. The Japanese market carries significant industrial, financial, and export exposure, while a global index adds a wider technology, healthcare, and consumer mix. Neither is "better," but they behave differently, and holding only one leaves you exposed to that one profile.
It is natural to overweight what you know. Familiarity makes home-country stocks feel safer, but comfort is not diversification — a portfolio of only familiar names can still be highly concentrated. Recognising home bias is the first step to sizing it deliberately rather than by default.
There is a legitimate case for some Japan tilt: if your future spending — rent, education, retirement — will be in yen, yen-denominated assets more directly match those liabilities and reduce currency mismatch. This is a reason to hold some Japan or some yen exposure, not a reason to abandon global diversification.
A widely used structure is a global (all-country) equity fund as the core diversification, optionally combined with a modest Japan tilt or a yen-linked sleeve to match home spending. This captures broad growth while acknowledging that you live and spend in yen.
Whichever you choose, remember the global option’s main added risks — foreign currency, geopolitics, and valuation dispersion — and the Japan-only option’s main risk — home-country concentration. The decision is about which risks you prefer to hold, sized to your goals, not about predicting which market will win.
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.