I would never have thought of asking my CC company what cover they provide - always thought the coverage was on the insurance policy you bought when renting the car.
The discussion above concerning credit card coverage is admittedly very US-centric. Are you from UK? If so, I'm not sure that UK credit cards come with these types of perks. But it's worth finding out. Also, when you rent a car in Jpn, the rental automatically comes with insurance coverages. What you have the option to purchase from a Jpanese rental car agency is not the insurance (which again is already included in the rental fee) but rather the exemption from having to pay the excess/deductible in case something happens.