By looking back now, it took me for 5 months to publish the first book. Japan ISBN agency makes a publisher's manual available, where we can find what to do when publishing a new book. It says donating the book to the national diet library is one of the "obligations". 

So instructed, I had to but it's still impossible for me to believe that the book of my creation to be in the collection of the national library. 

Lacking a sense of reality, I checked the donation procedure, and went to the library to donate. I had to go to the back entrance, not the main entrance for library users. When I told the guard "I came to donate a new book", he told me to write down my name and gave me a budge to wear in the building. I walked through the gloomy hallway, which seemed to me to be very much a back office of the library. 

Although publishers are obliged to donate one copy of every new publication, they can donate two copies so that the book will be available both at the main library in Tokyo and the branch in Kyoto. I donated two. It still feels like unrealistic, but I'm glad to fulfilled a duty.