Iman Valizadeh led Iran with 17 points and 3 rebounds, while Shogo Takata scored 21 points for Japan.
Iran are scheduled to meet Saudi Arabia and Qatar on Monday and Tuesday, respectively.
The top teams of four groups at the end of the Group Phase will qualify directly for the Quarter-Finals. The second and third-place teams will play in the Qualification to the Quarter-Finals Round with teams in Group A playing against teams in Group B while teams in Group C play against teams in Group D.
The teams that advance to the Semi-Finals will also qualify for the FIBA U17 Basketball World Cup 2024 to be held in Turkey from June 29 to July 7 next year.
TM/TT