Calculating gains, acquisition costs, loss offsets, three-year carryforward, NISA loss treatment, and multiple brokers.
A capital gain is sale proceeds minus adjusted acquisition cost; taxable listed-security losses can offset eligible gains and dividends and carry forward three years if you file, while losses realised inside NISA give no deduction.
A capital gain (譲渡益) is your sale proceeds minus the adjusted acquisition cost of what you sold, less relevant expenses. Acquisition cost includes what you paid plus applicable purchase costs, and when you have bought the same security at different prices, an averaging method determines the cost per unit. In a specified account the broker computes this for you; in a general account you must track it yourself.
Getting acquisition cost right matters because it directly sets the taxable gain. Errors usually come from forgetting reinvested amounts, multiple lots at different prices, or foreign-currency conversions.
Losses on qualifying listed securities in a taxable account can be offset against eligible listed-security gains and dividends. If offsets do not use up a loss in the year, you can carry it forward for up to three years to offset future gains — but only if you file the required tax returns for each relevant year. This loss relief is a real advantage of taxable investing.
Crucially, this does not apply inside NISA: a loss realised in NISA gives no deduction and cannot offset taxable gains or be carried forward. So you cannot tax-loss harvest in NISA, and loss-sensitive strategies belong in a taxable account.
If you use more than one broker, offsetting gains at one against losses at another generally requires filing a return that brings the figures together — the brokers do not net across each other automatically. The same applies to combining a withholding account with foreign or general-account results.
Keep your own records: annual transaction reports, acquisition-cost details, and dividend statements, retained independently of any single broker. These matter especially for general accounts, foreign accounts, departure from Japan, inheritance, and any home-country reporting. Do not assume a broker statement will remain accessible forever.
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.