I’ve used the same 350 ml screw-top stainless Thermos for years, mostly for barley tea. Tea stains built up deep inside; chopstick-sponge hacks couldn’t reach. I tried three brushes:

1. Marna stainless pot brush (~¥1,000) — looked sturdy, 35 cm long and 6 cm wide. Fit the bottle perfectly…until I pulled it out and got a faceful of water and suds. Back to the cupboard it went.
2. LEC “Geki-Ochi” bottle brush — “cleans with just water,” with cleaning beads and a removable head. The head kept loosening and popping off; cleaning was weaker than a normal melamine sponge.
3. Kokubo melamine-sponge brush (¥100 shop) — finally perfect. A rectangular melamine block scrubs sides, bottom, and even corners when you twist it. No water spray on removal. Sized just right for 350 ml bottles and still fine for 500 ml Thermos. Melamine shrinks with use, so I buy five at a time and swap.

Kokubo (small maker from Wakayama) supplies lots of 100-yen cleaning gadgets—you’ve likely seen their gear. This brush is the one I stockpile.

Kokubo: 100-yen sleeper-hit maker

• Founded in 1958 in Wakayama; ~68 employees; owns a pest-control sister company (Kiyo). They even have a tiny US office.
• Their products fill the cleaning aisle at 100-yen shops: vacuum nozzles, bottle brushes, melamine blocks. They also make a goofy lion-handled duster—I own it; it’s cute but not very grippy on dust.
• They run an online shop at list price (yes, the same items you see for ¥100). Treat it as a catalog, not a bargain bin.
• Kokubo leans into IT/marketing with a Facebook page and even publishes “life hacks” like “soak bark to make birdlime”-tier trivia. Often cringe, occasionally charming.

TL;DR: if you spot their melamine bottle brush at a 100-yen store, grab a handful before it disappears.