Is fishing in the river prohibited?

I travelled along sub-urban of Tokyo. I found some of nice river,Aiki river.

Can I do fishing in that river or I have to buy a license first?

And all the rivers are they free to do fishing?

Where can I find more information about fishing in japan.

Thank you in advance.

No fishing licence is required for freshwater fishing in Japan.
However, a fishing permit must be purchased to fish most lakes or rivers. The fee for the permit goes to the local fishery cooperative of each area.

Assuming you are talking about Aiki river in Nagano-ken, you need to contact the following fishing union of the area:

南佐久南部漁業協同組合〔小海町大字小海3981-1〕
TEL：0267-92-2167

or go to 長者の森 to buy 1,570 yen day ticket which is required for fishers.
http://vill.kitaaiki.nagano.jp/kankou/aikigawa/

By the way, I suppose some people call it "permit" while others call it "license".

I had not considered that for the fishermen I see using Tamagawa in Tokyo (a popular pastime), but I know other rivers do have restrictions on where you can fish or not - I would check with the local authority before heading off rod in hand.

Thank you for your all replies. Really helpful.

I had not considered that for the fishermen I see using Tamagawa in Tokyo

You do need to pay for a permit to fish in Tamagawa, and it can easily be done. Please refer to the following Japanese site where it says 多摩川の釣りで許可は必要？
https://tsurihack.com/1014

Actually, a great number of people openly fish illegally in various places throughout Japan. We often see huge (Japanese) signs warning them. But permits are sold in order to maintain not only the fishing industry but also the environment as well as your own safety, so you should always stop by to check about permission and other things you need to keep in mind when fishing at that spot. Any local fishing shop would be able to give you advise.

@ BEST79
That river was "Akikawa" in Nagano pref, really?
You saw in the upstream "Akigawa(秋川)" of Tamagawa(多摩川) name plates?
http://www.kensetsu.metro.tokyo.jp/ikimono2/images/suikei_tamagawa.jpg
http://home.s08.itscom.net/sekix/img/rs1492065928.jpg
If so, see the 秋川漁協(gyokyo) part of the table and their official web.
http://www.geocities.jp/f6kaneko/yuugyokenn.htm
http://akigawagyokyo.or.jp/index.php?%E9%81%8A%E6%BC%81%E6%A1%88%E5%86...
漁協 is shorten of 漁業協同組合(gyogyo kyodou kumiai) = Fisheries Co-operative Association.

Anyway, already answered outlines,
nearest fishing gear shop(釣具店 tsurigu-ten/釣具屋 tsurigu-ya) knows well.
If you can read Japanese well even if using translator with,
details are written in each prefecture's fisheries(水産業 suisan-gyo) page.
(if without maps, it may take time to understand)
This is a Kanagawa pref as sample, 遊魚(yuugyo) means "play fishing" by amatures.
http://www.pref.kanagawa.jp/cnt/f790/index.html
http://www.pref.kanagawa.jp/cnt/f790/p504690.html

River/Lake fishing ticket(pass) is called "遊魚券(yuugyo-ken)" or "遊魚証(yuugyo-sho)" usual.
https://goo.gl/f5yqUC
Also using another word "鑑札(kansatsu)" cases a lot. (depends on Gyokyo)
https://goo.gl/CN7ry3
一日券(=1日 ichinichi-ken)/日券(nikken) is 1 day ticket.(calendar date)
年券(nen-ken) is valid for all days of ON-season ticket.
(invaild for OUT/OFF-season = 禁漁期間 kinryo kikan)
半額券(hangaku-ken) is half price ticket for children or elder(depends on Gyokyo)
解禁/解禁日(kaikin/kaikin-bi) is start date of ON-season.
If don't buy it beforehand, must pay x1.5 or x2 usual to patrol staff. (this is not a fine as penalty)
Maybe already you know this year 2018 is 平成30年(Heisei sanjuu-nen).

If you want to know more about "River fishing", I can write more details well, is it necessary?
If so, pls tell me the specific season, area(or river name), kinds of target fishe you want to.
If you only need a method of searching,
it will come out someting info by searching simple words "River name and 釣果(chouka)" at Google.
釣果 is result of fishing, fr 釣り(tsuri/fishing) + 結果/成果(kekka/seika = results).

I do not know how much you are interesting in fishing now,
but I was able to introduce "Japan fishing show in Yokohama" if you asked me two weeks earlier.
http://www.fishingshow.jp/english2018.php

This exhibition is the largest in the Asia, only once per year.
It will be held in Osaka this weekend, a bit smaller hall.
http://www.fishing.or.jp/english/index.html
http://www.fishing.or.jp/index.html
https://youtu.be/P-lQ8BkUBXs
If you have time, I think that going to play is a lot of fun.
You may get excited that night and won't be able to sleep :)