S1·E89 Even It's in the Gaster-Water
Aired July 13, 1979
Synopsis
Shizuka accidentally swallows her mother's opal which is worth 500,000 yen. Doraemon and Nobita use the miniature sub from episode 41 to recover it from inside her, but play a trick on her which results in Nobita swallowing an eraser, which will have to be recovered.