Tokyo taxis are probably some of the most expensive in the world.  Fares typically start around 600-700 Yen for the first two kilometers, then increase by about 100 Yen for every additional 500 meters traveled.  The cost increases if the taxi is stuck in traffic and does not move for a lengthy period of time.  It also costs extra for late-night rides; expect to pay up to 30% more.

Planning on taking a taxi from the airport to downtown Tokyo?  From Narita, a taxi ride costs around 20,000 Yen (1-2 hours).  From Haneda the price is more reasonable; a cab ride will cost around 6,000 Yen.
 
Taxis can be found at taxi stands, hailed in the street or reserved by phone (although there is an additional charge for phone reservations).

A rental car in downtown is not advisable due to heavy traffic, confusing streets and high parking fees. In fact, most streets in Tokyo are not named; many are identified by numbers in each separate neighborhood.  However, if this does not deter you from self-drive transportation (or, if you plan on driving outside of the city through the scenic countryside), there are some regulations that must be followed.

First, an international driving permit is required.  (For visitors from France, Germany and Switzerland, an official translation of their driving licenses is also required.)  Permits can only be obtained in the home country, before visiting Japan.  Most national automobile clubs will offer this service.  While driving in Japan, the driver must have both the international driving permit and a valid driver’s license from his home country.

Japanese-owned rental companies such as Japaren, Nissan Rentacar, Toyota Rentalease, Nippon Rentacar and Orix Rentacar, offer cars in all sizes, including vans, SUVs and RVs in many cases.  International companies such as Budget, Avis and Hertiz, also operate in Japan, but their rates may be higher.

Rental costs vary from company to company.  However, expect to pay around 6,000 Yen per day, for the smallest-size car up to 15,000 Yen for full-sized cars.  A 1,000 Yen per day, insurance fee is mandatory.  Expect to pay more if you are renting during the busiest tourist seasons (spring and fall).

There have been many incidents of over charging from people taking taxis from Narita Airport into Tokyo. There are alternatives to taxis, such as the N'EX train or the Limousine Bus.