Baking soda and baking powder in Japan: what it's called and where to buy it
Baking soda is 重曹 and baking powder is ベーキングパウダー. Both are small packets on the baking shelf.
Look for this on the label
重曹
jūsō · baking soda
ベーキングパウダー
bēkingu paudā · baking powder