It’s still best to air on … It’s better to wear long-sleeve shirts or put a jacket/coat in October and November. In September, only T-shirts are enough in the daytime, but you may feel a little cold in early morning and at night.
Here's What to Pack for Japan in Every Season! About September and October Weather in Kansai It will still be warm in September, so you can essentially wear summertime clothing.

However, the mornings and evenings might be cool, so come with long-sleeved shirts or light cardigans.

Northern Japan including Hokkaido and Tohoku region gets cold earlier than other areas. In October, long-sleeved shirts are ideal for sunny afternoons. You may feel a little cold in early morning and at night or on a rainy day in June while really hot in the daytime.

To stay warm, they may need to layer wearing long-sleeved T-shirts, a sweater, warm socks and a down coat, and use heat packs alternatively. What to Wear in Japan?
Others who live in cold climates and used to that low temperature, just T-shirts and a jacket or a hoodie may be enough even in winter. It’s highly recommended to put a light cardigan on over your T-shirt in June. You would need a coat in the late November. What to wear in Japan The Japanese are open-minded about fashion – you can wear pretty much whatever you feel comfortable in, including shorts and tank tops as long as they’re not too revealing (and the weather permits). In September, there will plenty of warm afternoons when you can wear short sleeves. Summer is the hottest season in Japan, nevertheless, there is a large temperature gap depending on the conditions. Bring a sweater or a hooded jacket for cold mornings and evenings. For shoes, you will see a lot of people in sandals. What to wear. For a complete packing list and capsule wardrobe for Japan in autumn check out our awesome Japan … Mind the Rainy Season! (2020 Edition) Spring in Tokyo: Weather and What to Pack in March, April, and May. You should put on thin, light and breathable clothing. What to wear in Fall (September, October, November) In Fall there is less rainfall during the fall months, but a rain jacket is still advisable. What to wear in Japan in summer. Dress in merino wool layers.