The first Deity should be enshrined in the oldest shrine
In Japan it is said there are about 80,000 shrines. I thought the history of the first shrine started when the first deity was enshrined. But how can I examine? there is a good history book in Japan called "KOJIKI (Japanese myth)". Amaterasu is the deity who rules all other myriads of deities. Then, ISE JINGU the first shrine enshrined Amaterasu is the oldest one? No, Amaterasu has parent deities. Izanagi and Izanami.
"Taga of Oumi" where retired Izanagi spent his life.
It is well-known that Izanagi and Izanami are the Earliest ancestor of deities. If so, it is natural to think the shrine enshrines them is the oldest shrine. There are so many shrines enshrine them, a famous one is IZANAGI JINGU in awaji island in Hyogo prefecture. According to Kojiki and Nihon Shoki (another Japanese myth), after Izanagi transferred power to Amaterasu, he retired in the place called "Taga of Oumi [淡海]". It is said the place in Awaji island. Speaking of Awaji Island, it is also the the first island made by Izanagi and Izanami's "Kuniumi (creation of Japan islands)". Even in IZANAGI JINGU website, it is written that this shrine is the oldest shrine in Japan. If so, I would say the research is done, but there is one more famous shrine enshrines Izanagi and Izanami which is also located in the place called "Taga no Oumi" in Shiga prefecture (used to be called Oumi [近江]). It is called TAGATAISHA shrine.
Oumi (淡海) and Oumi (近江)
Pronunciation 淡海 and 近江 is both "Oumi", actually this is about Japanese character, so it is a bit difficult to explain to who does not use it. In any case, the question is which is the oldest one. In the Middle Ages, TAGATAISHA was more popular for worshiper than IZANAGI JINGU. TAGATAISHA was thought as set with ISE JINGU for their tour of worship. Furthermore, Takeda Shingen (famous samurai in 15th century) and Toyotomi Hideyoshi prayed at TAGATAISHA. Therefore TAGATAISHA is the shrine Takeda Shingen and Toyotomi Hideyoshi admit, you will be convinced to believe TAGATAISHA has much more credibility. About 1.7 million people a year currently visit the TAGATAISHA, also it was very popular as a shrine worshiping Izanagi and Izanami in the Middle Ages as well.
Which is old after all
Personally I like the story "Izanagi chose to spend his life in the island which he created with his wife first of all islands." I would say IZANAGI JINGU is the oldest one. I would like to visit both of the shrines someday.