First, you need know to how to calculate Japanese inheritance tax and how to evaluate the taxable value for each asset categories.
Cash and bank balance are the worst. They are evaluated on their face value. If he had 100 MM JPY, it is to be assessed as 100 MM as taxable estate.
On the contrary, the best is real estate. They are not to be assessed to the current market price. They are assessed at about 60-70% of the market value. Even better, if the property has a tenant, the valuation of the land will be reduced to a half and the building portion 70%.
If you are going to live there, it will be 80% discount.
This I guess is one of the reason why the price of “tower mansions” has soured a lot in the past decade. Japanese Inheritance Tax Law was revised in 2015 to lower the base deduction so that every body who has estate more than 30MM + 6 MM * number of heirs are now taxable.
Second, the taxable valuation is depended on who take estate. His spouse is the best destination because she will be exempt up to the greater of 160 MM JPY and 50% of his total estate.