Assuming that this translation is correct, Article 10(viii) identifies photographic works as "works". Article 13 described works not covered (basically, legal documents of the government), and nothing resembling "campaign posters" is included. Article 21 states that the author has exclusive right to reproduce his work. There are no indications that posters used for political purposes, including electoral politics, are excluded from copyright protection. There is in Article 40 permission to "exploit, by any means, political speeches delivered in public and speeches delivered in the course of judicial proceedings" or "speeches not falling within the preceding paragraph, which are delivered in public by organs of the State or local public entities", but these fall under the penumbra of government documents.
Article 41 says that
For the purpose of reporting current events by means of photography,
cinematography, broadcasting or otherwise, it shall be permissible to
reproduce and exploit a work involved in the event or a work seen or
heard in the course of the event, to the extent justified by the
which might maybe cover your case. There are other government-action type permissions such as the Chief Librarian of the National Diet Library to archive information, which presumably are irrelevant.
The specific permission that you mention, Article 46, says:
It shall be permissible to exploit artistic works permanently located
in such open places as mentioned in paragraph (2) of the preceding
Article and architectural works by any means not falling within any of
the following items:
(i) multiplication of a sculpture and offering it to the public by
transfer of ownership of its copies;
(ii) imitative reproduction of an architectural work and offering it
to the public by transfer of ownership of its copies;
(iii) reproduction of a work for the purpose of locating it
permanently in such open places as mentioned in paragraph (2) of the
(iv) reproduction of an artistic work exclusively for the purpose of
selling its copies and sale of such copies.
where the places are "in open places accessible to the public, such as streets and parks, or at places easily seen by the public, such as the outer walls of buildings". The ordinary interpretation of "artistic works" would not apply to a campaign poster, and at any rate they are not permanently located in such open places.
So, there is no political poster exception.