I'm not sure how hot it was that day in Japan but I've ridden my bike in the Mojave Desert area of Southern California where in the summer the temps get over 100 degrees F.
I would also spray myself when I got too hot with water from my bottle, but I had to carry a Camelbak 70 ounce bag on my back.
Now they have these scarfs that you get wet and they will keep you about 5 to 8 degrees cooler by simply laying it around your neck, and they have the same type of thing you put on your head then put your helmet on, I haven't tried one of those yet.
I just had to make sure I stayed hydrated. I also found that loose fitting white colored clothing to works the best, then I found a jersey with a bunch of tiny holes that is the coolest jersey I have ever worn including a high tech $135 jersey, and the one I bought with the holes was just $12!