If the engine has not been started
JAF advises drivers not to start the engine or switch the power on after filling with the wrong fuel. The tank normally needs to be drained by a repair facility.
Do not attempt to move or drain the vehicle yourself at the station. Tell staff which pump and fuel you used, approximately how much was added and the vehicle model. If it is a rental car, contact the rental company immediately.
If the engine was started
Wrong fuel can cause loss of power, unusual noise or smoke. If you can stop safely, stop and do not restart. Continuing to drive can increase damage.
JAF notes that draining the fuel may not be enough after operation; filters, lines and injection components may also require inspection or repair.
A Japanese kei car does not use diesel
The word kei in kei car refers to the vehicle category, not diesel fuel. Most kei cars use regular petrol. Always check the rental explanation, fuel-door label and vehicle manual rather than guessing from size.