I found the reply myself on ANA's homepage, quoted below

Passengers who transit to/from International Flight

You don’t need to enter Cambodia.
After arriving, please proceed through the corridor, at the side of staircase to Ground Floor, to the International Flight Transit Counter. If you had not checked in for a connecting flight, proceed to the gate for a connecting flight through a transit security check after checking in at the International Flight Transit Counter.
*Please contact to a security staff, an airport staff or an ANA staff if you can’t find any staff at the transit counter.
*You need to enter Cambodia and check in at the counter in a departure lobby for connecting to/ from some airlines’ flight. Please check the condition on your own responsibility.