Your frame was manufactured by Miki of Japan in 1985. While Miki was a contract manufacturer of Sekai frames, they also manufactured several other brands. I see that you are located Indonesia so I strongly doubt that this is a Sekai brand ATB. Sekai started out as a USA brand but by 1985 had been taken over by Norco of Canada, who marketed Sekai only in the USA. While it's possible that a non-related Sekai brand existed elsewhere in the world, I haven't seen any cases of foreign market Sekai. More likely, it is some smaller, possibly local, Asian market brand that was contract manufactured by Sekai.