I've spent a lot of time in different libraries all around Tokyo
I just saw a foreigner here get asked to put his laptop away in the library and he looked really really confused. I told him what's up but i feel like others here should know as well: Not all libraries in Japan allow laptop usage.
Okubo Library (which is where this happened) is a good example of a library that doesn't allow it. The general rule is that if there are no power outlets where the desk is, then laptop usage is not allowed there. And in Okubo library, there are no desks with power outlet anywhere. I've been to other libraries where this was the case.
Typically, most libraries do allow laptops but smaller ones don't so it's good to check with staff if you don't see any power outlet.
Additionally, in certain libraries, you need to make an appointment to use certain desk. Like in Shibuya Central library where you gotta make an appointment to use certain desks with power outlet. Also most libraries that do allow laptop usage only allow it in certain sections of the library. It's not like in America or Canada where you find any desk and then pop open a laptop.
Tbh, a lot of library rules in Japan are kind of unnecessary and sometimes the staff will turn a blind eye even if there are rules against it tho. But this is just how they do things here so it's good to know so you don't get confused
by netro008