I'm Myanmar passport holder and flying to Myanmar from Mexico but I have to transit in Japan. I will arrive at Narita Airport Terminal 1 at 6:20 am and depart from Terminal 2 at 9:15 am on the same day, 2 hours & 55 minutes in Narita. I can show my flight ticket onward, and I'm just going back home. Do I need visa in this case? I've called the Japanese Embassy in Mexico & they said I might need visa but might not if I don't leave the airport.
If this is just a layover between two flights with a single airline, then your luggage should automatically get transferred and this page on the Narita Airport website confirms that going from Terminal 1 to Terminal 2 consists only of a standard security check for you and your carry-on, and taking an airside shuttle bus to the new terminal. It never mentions going through customs or immigration. You should not therefore need a visa, since the whole transit is airside not landside.
This TripAdvisor forum discussion also confirms this indirectly; the discussion about movement between terminals is purely concerned about whether you can ride the bus without a ticket for a flight from the other terminal, and never mentions needing to get a visa for the transfer.
Disclaimer: If this is a transfer between two airlines, then you'll have to claim your luggage and I'm not sure whether getting back into Terminal 2 will be possible without going outside the airport, thus necessitating a visa. From the map on page two of http://www.narita-airport.jp/files/official_guide_t1_02_en.pdf, it seems you can get your luggage and then go catch the shuttle, but it's a little difficult for me to tell for sure.
You do not need a visa, according to IATA Timatic, used by airlines and travel agents to verify passengers travel document requirements for their destination and any transit point. Whether your luggage is handled by the airline(s) or by you, you are able to Transit Without Visa (TWOV) as a you have an onward ticket and are transiting on the same calendar day.