Hmm...it's unusual that you got a Taiwanese adaptor but a Japanese Famicom, but I guess it's possible. And there actually may not be a problem with the output from the adaptor to the Famicom. If you compare the Japanese and Taiwanese Famicom outputs, they're both DC10V 850mA, + ------ C------- -. The only problem then is the input from the outlet to the adaptor, which is a +10V into the adaptor, again with the Famicom having a voltage regulator to help cope.

I use a converter and will alway recommend using one -- though the Famicom can handle extra V, it wasn't built for a consistent or constant input of extra V, I'd imagine. I don't know how easy it will be to find a converter for a 10V difference.