Yes, it is generally possible to leave the airport during a 20-hour layover in Japan, depending on various factors. However, there are a few important points to consider:

1. Visa requirements: Check whether you need a visa to enter Japan. Citizens of certain countries may be eligible for visa-free transit or may require a transit visa. Make sure to review the specific visa requirements based on your nationality.

2. Immigration procedures: Upon arrival, you will need to go through immigration and customs. Factor in the time needed for these processes, as they can take a significant amount of time, especially during peak travel periods.

3. Distance from the airport: Consider the distance and transportation options between the airport and the places you wish to visit. Some airports, like Narita International Airport or Haneda Airport in Tokyo, have good transportation connections to nearby cities, making it easier to explore during a layover.

4. Time constraints: Keep in mind the time it takes to travel to and from the airport, as well as potential traffic or transportation delays. Leave sufficient time to return to the airport well before your next flight's departure.

5. Luggage storage: If you have checked luggage, check if there are luggage storage facilities at the airport or nearby, so you don't have to carry your bags during your layover.

Always check with your airline and relevant authorities for the most up-to-date information regarding layovers, visa requirements, and any specific regulations or restrictions that may be in place.